I think most opponents are not questioning the possibility of hacking mail in election systems. I don't know if the idea you posted would have worked because lots of people can spitball ideas without fully thinking them through. Even so, proponents contest the likelihood of that happening balanced against other considerations, like the disenfranchisement of those who cannot or will not reach a polling place for health reasons.

There is also the question of differential susceptibility. Sure election fraud is possible with mail in ballots. I'd even stipulate that it's easier than casting fraudulent in person votes. But I think it remains to be answered how easily it can be scales, and whether the rewards are worth the risk this would pose to perpetrators' freedom.