Negatives cancel, so I think you cancel the first 2 and get "this is to say that some companies are pathologically incapable of delivering software", which reads a bit nicer

Generally that works but sometimes it doesn't.

For example, some people say statements like "I don't want no towels".

What they really meant: "I don't want towels"

What I get if I cancel the negatives: "I do want towels"

The meaning flipped :)

Double-negatives in the "not+no" formulation tend to be a special case. The easiest rule (as you realized) would be "If there's a 'no' before the noun (direct object?), in an already negated phrase, then ignore the 'no'".

It's technically incorrect but common English.

That is incorrect usage though; every rule 'generally works but sometimes doesn't' if we account for people not observing it.

> That is incorrect usage though

We don't need no education

You wouldn't be here quoting that line if it were correct.

But pointing out that language rules aren't something that people strictly adhere to, with rigorous logic, is a useful observation.

Undoubtedly, but it was a response to someone explaining how to use English (correctly), not how to understand someone else using English (possibly incorrectly).

If I say 'proper nouns have a capital first letter' I don't think 'actually oliver sometimes people don't do that' is helpful.

Point. Edited and hopefully clearer. I try to keep non-first language readers in mind, but vernacular crept in before a second cup of coffee.

IMHO, "this isn't to say" is a common expression in English, so I didn't register it as a negative. But you're absolutely correct.