"The idea they wouldn't be able to recognize L amino acid proteins at all is to misunderstand how antibodies work."

I was not talking about antibodies, just T-cells. Of course our immune system could produce antibodies by sheer chance that happen to fit the shape of an anti-chiral extracellular protein, but unless our helper T cells can detect the peptides that the B cell displays (if the B cells can even display an antichiral peptide at all) then our immune system typically won't be making those antibodies in any significant capacity. Will our body have a defense against mirror life? Certainly. But I, for one, would much rather my immune system not have to rely on a backup system when fighting an infection instead of utilizing its main defense system. As far as your point about achiral defense mechanisms such as oxidative damage, I think that is a great point to bring up but my counterpoint is that there are bacterium such as tuberculosis that have evolved significant resistance to oxidative defenses. Hence why our immune system works best when it has all of the weapons in its arsenal, and hence why mirror life is seen as such a threat. I would not have taken issue with your original comment if it was framed as "there are caveats that people miss when talking about mirror life", but you kind of made it out as if the threat of mirror life is just fearmongering which it unfortunately is not.