How does 'That it be' mean 'Yes it is'?

Am I right that https://english.stackexchange.com/a/96966/50720 is claiming thus (as regards the title of this question)? If so, how do you derive or explain this equality of clauses?

I'm also reminded of Edmund Blackadder being irritated by an old witch's turn of phrase, and reprimanding her:

"It's Yes it is not That it be!" http://www.bbc.co.uk/programmes/b006xxw3

For anyone who doesn't know, this is a BBC comedy programme and the line is obviously done for comic effect, but it stems from the fact that 'it be' was commonly used at one time, and might still be used in certain English dialects.

It's clear that that many kinds of 'incorrect' or non-standard usage have been around for a long time and I don't think it's up to us (we) in the 21st century to tell them they were wrong. The point of language is communication. As long as there's no chance of confusion when using Not I/Not me, it's a matter for personal choice. Let it be!

Original context:

Blackadder: Tell me old crone, is this Putney?
Old woman: That it be. That it be.
Blackadder: "Yes it is", not "That it be". You don't have to talk in that stupid voice to me, I'm not a tourist.

Solution 1:

It is an old and dialectal form. Is it raining? That (i.e raining), it be. This means, in modern Received English Yes, it is.

In the Norfolk dialect today that nearly always replaces it, where it would otherwise be the subject of the sentence or clause.

How far is it to London? That's over 100 miles.

Extract from Wikipedia entry re Norfolk dialect.

The word that usually denotes it when it is the subject of the clause, so that "it is" becomes "that is" and "it smells funny" becomes "that smell funny".[16] This does not imply emphatic usage as it would in Standard English and indeed sentences such as "When that rain, we get wet", are entirely feasible in the dialect. (Incidentally, 'it' is almost never heard as the first word of a sentence in the speech of a true Norfolk dialect speaker, e.g. "It's a nice day today" is virtually always rendered by "Thass a nice day today".) It however, is used for the direct and indirect object, exactly as in Standard English, cf. "When that (subject) rain, I don't like it (object)"/"I don't like it (object), when that (subject) rain" Wikipedia - Norfolk Dialect http://en.wikipedia.org/wiki/Norfolk_dialect

Solution 2:

Be is often used in dialect where standard English uses is. It is not correct, as Blackadder acidly points out.

But "That it is" would be perfectly grammatical, though a little unusual. "So it is" is amother way of expressing agreement and is very common, though it implies surprise as well.