[tripletao]

Exactly. The paper linked in the grandparent is questioning the exact date of reintroduction, not whether reintroduction occurred. The usual guess seems to be

> the result of vaccine trials in the Far East involving the challenge of several thousand military recruits with live H1N1 virus (C.M. Chu, personal communication)

https://www.nature.com/articles/nm1141

That part is genuinely uncertain though, and probably unanswerable. Historical surveillance was weak, and those who do have information may not wish to implicitly confess to hundreds of thousands of deaths.

No one is seriously questioning that it spent ~20 years in a lab freezer. There was some speculation about virus frozen in the Arctic or such; but since that's never been observed to happen any other time, and multiple labs were known to be working with frozen and thawed virus, I think that's pretty abandoned.