It's interesting to wonder why only 5M accounts were affected by this exploit, especially if it's brute forceable. IIRC this vulnerability was widely known about for at least months before it was fixed, so I can't imagine nobody in the know had access to the resources/botnets necessary to enumerate through every account.

Have only 5M accounts linked their phone numbers on Twitter? That's less than 2% of their total accounts (~290M). I don't know what the industry average is for linking phone numbers, but this seems like an exceptionally low ratio.