Did the flu actually disappear during the pandemic?

Flu did not cease to exist, but reported influenza activity fell sharply in many places during the first pandemic seasons. That is different from saying there were no flu infections anywhere.

Reduced travel and mixing, school closures, masking, and people staying home when sick could all have lowered transmission. Changes in testing and surveillance may also affect how many cases were recorded, so the observations alone do not tell us exactly how much each factor contributed.

The claim that precautions played no role goes too far, but so does treating them as the only possible explanation. Several changes happened at once, and it is difficult to separate their effects from surveillance differences or other shifts in how people moved and interacted.

A careful summary is that influenza circulation dropped substantially in many regions during that period, not that flu was imaginary or literally gone.