This sounds as outlandish as advising to dunk a baby in bleach after birth.
The only reason I can think of for this myth to have spawned is the assumption that bleaching the plant might offer any protection against aggressive pathogens – protection that would dissipate after the bleach dries and kill any beneficial bacteria. Maybe at some point, commercial growers tried this out to prevent viral transmission between plants. But, who knows, it showcases people's lack of general understanding of microbes.
There's something to be said about using a good probiotic. Going back to the baby analogy, evidence suggests that children exposed to germs grow more robust immune systems. In fact, some studies point to children who grow up on farms or have pets being less likely to experience immune-related illnesses.
The hygiene hypothesis: How being too clean might be making us sick - Vox
Not only that, but if you were born before 1990 in a foreign country (and your parents weren't knuckleheads), you might've received the BCG vaccine a few weeks after being born. Some studies suggest the vaccine boosts innate immune mechanisms (trained immunity) for an undefined period of time – in addition to accomplish its intended protection against TB.
Bacterial and fungal inoculation wouldn't necessarily apply to the concept of trained immunity, since inoculation aims to foster a symbiotic relationship and not to train the plant's immune system. However, if the colony is maintained and the symbiosis successful, who's to say that couldn't potentially help the long-term health of these seedlings.
I currently alternate between Quantum Total (Ray, above, sells it) and Inocucor Garden Solution. Inocucor is hard to find nowadays, but a product called BioAG from SCD Probiotics seems to have a very similar composition. A product that I haven't tried yet but that retails at a not-terrible price and seems to include a good amount of mycorrhizae is Plant Probiotics from Micra Culture. (This is all assuming labels tell you anything, which they mostly don't).
All of the above include Bacillus subtilis which antagonizes and can kill several strains of Erwinia. Quantum and the Micra Culture products also contain Bacillus amyloliquefaciens, which is sold and used as a "bio" fungicide to fight several pathogens. Many of the additional strains are nitogen fixing bacteria that help the plant process nutrients in exchange for carbon. I'm sure your newly deflasked orchids would be in good company with either of the above.
I'm not deflasking orchids, but every plant I propagate is sprayed with a solution containing Quantum and Kelpak and I can't complain.