While I don't see the answer there as condescending or assholish, it's not clarifying. The question was if it's about one source, or many sources and the answer doesn't even have the word source in it.
It would be way clearer if the answer was something like this:
You stop taking damage over time when you change from taking damage over time to not taking damage over time. So that means you can't be taking any damage over time from any source for at least a tick.
The first part (existing in the answer) is the technical explanation of the condition. It says that it's there whenever a variable changes state. But what's missing is the clarifying part. The part where it answers when exactly that state change occurs, best if using a similar language to the question.
That clarification was present in the actual post - there's an fourth question and answer in this section specifically about multiple sources of damage over time directly following this which have been omitted from the screenshot. The answers here seem incomplete because they are.
(there's also a fifth part clarifying that losing life over time is not taking damage, but that's not relevant to this specific conversation).