We really can't. Militaries in particular aren't into bioweapons. Too hard to aim. They're pretty much strictly the domain of terrorists, and terrorists don't have the resources to execute attacks on the bleeding edge of biotech. Remember, no one has built a regular cell from scratch, much less come close to building a mirror cell.
Military labs do work with bioweapons, and (unsurprisingly) they sometimes have trouble containing them just like regular labs. E.g. https://en.wikipedia.org/wiki/1971_Aral_smallpox_incident https://www.fredericknewspost.com/archives/what-went-wrong/a... https://www.nytimes.com/2004/05/25/world/russian-scientist-d... https://www.usatoday.com/story/news/nation/2015/07/23/army-a...
For defensive purposes, FWIW. I suppose they might create some mirror molecules to test defenses, but I believe gus_massa's point stands that you don't need to generate full, functioning cells for that purpose.
The full report is here: "Technical Report on Mirror Bacteria: Feasibility and Risks" https://purl.stanford.edu/cv716pj4036 The premise reminds me of the "Rifters" trilogy by biologist and science fiction author Peter Watts. In it, an archaic deep sea microorganism "ßehemoth" that outcompetes all other kingdoms of life is brought to the surface and wreaks global havoc as it spreads. https://www.rifters.com/maelstrom/…
I want to thank you for this comment, I went out and grabbed a copy of Starfish and just finished it, what a ride!