It's not a conspiracy theory at all. It's the sudden addition of protection from flu to justification to anti-covid measures.
We didn't have this "you should follow masking and social distancing because of the flu as well", "everyone should be vaccinated against the flu" rhetoric before. It's literally just popped up as people are asking the public to maintain covid restrictions.
It's fairly obvious that where normal flu wasn't mentioned before, then when covid starts getting better people start adding flu to reasons for following covid restrictions, the thing they want is the covid restrictions and they're looking for reasons to maintain those restrictions, not to protect people from flu.
It's extremely sensible to question this.