I’ve read similar claims before. I don’t know enough about viral contagion to know how risky this is but on the surface it sounds logical.
I think about those superbugs they find in hospitals. Under normal conditions those most dangerous spores are kept to a minimum by the presence of other bacteria. But when you sanitize things so well you kill 99.9% of all the bacterial colonies, you have just created lots of space for the most deadly 0.1% of spores which cannot be killed through sanitization to grow wild.
Analogy may not be perfect but something like that.
The "vaccine escape" case is being covered here. The analogy to bacteria is taking a sub-therapeutic dose of an antibiotic. It kills some of the infection, leaving some in place that is resistant to the antibiotic. Eventually it selects for an organism fully resistant to the antibiotic. The mRNA "vaccine" selects for SARS-CoV-2 that match the sequence of spike protein on the mRNA. A variant that doesn't match "escapes" and continues to replicate and can be transmitted to other persons. Others infected by this "escaped" mutation will get sick whether "vaccinated" or not.
SARS-CoV-2 will continue to mutate with or without the "vaccines". In time, they will mutate enough to make the "vaccines" ineffective. The same phenomenon observed with flu viruses.