Primary determinant for deciding if you should use ‘the’

In the following dialog, both

‘the people who we don’t know’ and
‘Ø people who we don’t know’

might mean either

‘all the people who we don’t know’ or
‘some of the people who we don’t know.’

‘The people who ...’ sounds more like ‘all’ than just ‘Ø people who ...’ according to some native speakers. However, it is IMPLIED/ FELT/ PERCEIVED.
In fact, some native speakers say ‘the people who we don’t know’ sounds no more than just ‘some.’ ‘All or some’ shouldn’t be the determinant.

What do you think is the determinant, then? Any and all insights would be welcome. What do you think is the determinant for deciding you should use ‘the’ or not in the following example? Some people feel you mean ‘all the people’ with ‘the,’ but it’s not the primary determinant, many people think. Would it be your mind that wants to emphasize the meaning of the relative pronouns that places ‘the’ there?

(Suppose there is no context, explicit or implicit, that brings on the definite article in front of ‘people’ and ‘ones’ other than, possibly, the relative noun clauses ‘who we don’t know’ and ‘who we know.’)

Tom: Although the/Ø people who we don’t know from our Sendai factory came to the party, the/Ø ones who we know didn’t.

Sam: Oh, I didn’t even know you have a factory in Sendai.

(I wrote this dialog, thinking ‘we’ are Tom and some of his co-workers, and that it doesn’t include Sam. Sam is from outside Tom’s company)

Starting from basics:

the is the definite article (singular & plural)
a is the indefinite article (singular); for plural indefinite, no article is used.

So:

The man/men refers to a specific/defined (or definite) man/men (whom you have already specified or subsequently specify); e.g.:
- The man with the dog didn't clear up the dog mess.
- The men mending the road did a very good repair.

A man / Men refers to an unspecified/undefined (or indefinite) man/men (the actual man/men is/are not specified/defined; e.g.:
- I saw a man walking his dog today.
- There were men mending the road outside my house.

Likewise:

The people refers to a defined/specific (or definite) group of people
People refers to any undefined/unspecific (or indefinite) group of people.

Back to the phrases in your question:

the people who we don't know from our Sendai factory came to the party

refers to a specific defined group of people, namely the people from the Sendai factory who you don't know.
In that this is a specific defined group, it necessarily implies all of the people who you don't know.
So, everyone from the factory that you don't know came to the party.

And similarly:

the ones who we know didn't.

means all of the ones you [do] know didn't come.

On the other hand:

people who we don't know from our Sendai factory came to the party

refers to an undefined/indefinite group of people, namely [some] people from the Sendai factory who you don't know. (It could be all of them - but that is not specified.)

And similarly:

ones who we know didn't.

means some of the ones you [do] know didn't come.
[In practice, in English you wouldn't say "ones who we know"; you would simply say "some who we know".

Summarising:

because the is the definite article, it does mean all of the defined group.
because the absence of an article with a plural noun is equivalent to the indefinite article with a singular noun, it need not mean all of the group and hence implies some of the group.

Thus, far from your suggestion that whether the presence or absence of the causes it to mean some or all is merely 'implied, felt, or perceived', it is actually an issue of fundamental grammar.

As a rough guide, when there is no determiner, this tends to indicate that the entity/entities involved are indeterminate: in other words, that the speaker does not see them as a 'list of specific things/people that could be listed/named/individualised'.

So for example in these cases:

(a) The/some people we had met before came to the party. (They were Jim, Bill, Karen, Anne...)

(b) The/some people we didn't know came to the party. (There were 20 of them. We later learnt they were called Sam, Brenda, Mary...)

It would potentially make sense to add the part in brackets, 'specifying' the group of people. But in the following cases, adding the bit in brackets sounds slightly odd:

(c) People we didn't know came to the party. (??There were 20 of them.)

(d) People we knew came to the party. (??They were Brenda, Alan and Dave.)

So when you use the 'bare' noun without the determiner, you are indicating that the group/entity is 'non-specific'. Because of that, it often sounds pragmatically odd if you go on to specify them as in the parts in brackets in (c) and (d), and if you added the bracketed parts in (c) or (d), it would sound more natural to use the determiner in the first sentence.