I am not a crytographer (by any means!), but my understanding from conversations with folks in that space is that it's very possible for a bad PRNG to generate

I am not a crytographer (by any means!), but my understanding from conversations with folks in that space is that it's very possible for a bad PRNG to generate primes that pass the standard tests but are still very breakable. I really don't have any more information than that; I should read up more there. Either way, that's by no means the biggest issue here -- some browsers already have CSPRNGs -- but rather that the code is coming from their server and is considered to be compromised by default. Browser-side crypto without very strict security boundaries on key generation and access is just bad news.