I think the risk of infection to humans would be very low. The human body can generate antibodies to pretty much anything (including PEG, benzenes, which never appear in nature) by selecting protein sequences from a huge library of cells. This would activate the complement system which targets membranes and kills bacteria in a non-chiral way.
The risk to invertebrates and plants might be more significant, not sure about the specifics of plant immune system.
I think it’s very unlikely that a mirror bacterium would be a threat. <1% chance of a mirror-clone being a meaningfully more serious threat to humans as a pathogen than the base bacterium. The adaptive immune system just isn’t chirally dependent. Antibodies are selected as needed from a huge library, and you can get antibodies to loads of unnatural things (PEG, chlorinated benzenes, etc.). They trigger attack mechanisms like MAC which attacks membranes in a similarly independent way.
In fact, mirror amino acids already somewhat common in nature! Bacterial peptidoglycans (which form part of the bacteria’s casing) often use a mix of amino acid in order to resist certain enzymes, but bacteria can still be killed. Plants sometimes produce mirrored amino acids to use as signalling molecules or precursors. There are many organisms which can process and use mirrored amino acids in some way.
The most likely scenario by far is that a mirrored bacteria would be outcompeted by other bacteria and killed by achiral defenses due to having a much harder time replicating than a non-mirrored equivalent.
I’m glad they’re thinking about this but I don’t think it’s scary at all.