This is British slang, so you don't really hear it in America. And you're right, it does mean, "Who are all these people?" But it's quite informal. For example, if you were in a building and saw a bunch of government officials in suits, you probably wouldn't say, "Who are all this lot?" But if you walked into a nightclub and you saw a bunch of guys who were dressed poorly, you could say, "Who are all this lot?" Yes, it's grammatically incorrect, but, as you know, that's the nature of some slang. Hope this helps. Good luck!

yep thats the British informal a particular group of people or group of things. "where has all this lot come from?" :)

Thank you, Małgorzata! :)

Or maybe it's a matter of context, just like in saying "He's good people", which is usually used in movies, tv series and so on by lower classes - intentionally grammatically incorrect

In this case "lot" is a singular colletive noun, which means that as a noun it is singular but semantically it refers to collection of people. So "are" is about people (plural) and "this" is like "the" before singular noun. I am not 100% sure about this, so I recommend you to read about collective nouns.