Why does this sentence use "does not" instead of "do not"? [duplicate]

I have two sentences that I was unsure if I needed to use "do not" or "does not". Now I know that the correct one is the one with "does not" but I don't understand the reason why. The two sentences are as follows:

Nevertheless, having those skills does not automatically guarantee their performance as marketers.

Nevertheless, having those skills do not automatically guarantee their performance as marketers.

The rules is that you use "do not" with words that are plural and "does not" with words that are singular. In my mind, "those skills" is "they". And they do not.

I don't understand why "those skills" is "It". It does not.

Someone told me that the reason that "does not" is the correct option is because of the word "having". But I don't understand the reason or if there is a rule. Is this correct? If so, why? And if not, can anyone please explain? Thank you.

It is the state of having those skills that is the subject of does. Since that is singular (the state of having or not having), then the verb has to match -- does.

If still in doubt, try swapping the subject out for something else:

Insanity does not automatically guarantee their performance as marketers.

or

Being John Malkovich does not automatically guarantee their performance as marketers.

Similarly,

[Having those skills] does not automatically guarantee their performance as marketers.

Does that make sense?

If the action that were (not) taking place was (not) being done by the skills, then it would be "do not" e.g. "The skills do not automatically guarantee the person's performance as a marketer."

In this case, the person is the one doing something, namely "having" the skills. Let's expand your sentence into what it means: "A person having the skills required for marketing does not automatically guarantee their performance as a marketer".