How correct was Justin Trudeau's explanation of Quantum Computing?
Trudeau's explanation wasn't wrong, but it also wasn't very informative.
Scott Aaronson, a well-known quantum computational complexity theorist, made a blog post discussing it:

What marks does Trudeau get for this? [...] Trudeau doesn’t really assert much here: basically, he just says that normal computers work using 1’s and 0’s, and that quantum computers are more complicated than that in some hard-to-explain way. [...]
[...] I’d grade Trudeau’s explanation as substantially more accurate than what you’d get from a typical popular article. [...] clearly suggests that he knows it’s more subtle than just “0 and 1 at the same time,” and he also knows that he doesn’t really get it and that the journalists in the audience don’t either. [...]
[Grade: B+].

There's a lot of discussion in the comments of that post.