Benzyl alcohol is a bacteriostatic, not a bactericide. In other words it prevents growth, but does not kill bacteria.
Not much growth is possible anyway in an anabolic steroid oil formulation because the amounts of nitrogen, phosphorus, etc are virtually zero.
You are correct that on injecting air into a vial there is the possibility that an airborne bacterium may be injected into the vial.
But under ordinary living conditions (maybe not in a pigsty) this isn't a problem with oil formulations that lack BA.
As for a dirty stopper, indeed one should follow good practice of fairly thoroughly wiping the stopper with an alcohol wipe before inserting the needle.
It probably is not the case that any substantial portions of injections are actually sterile in the literal sense. It takes a lot of work to actually sterilize the skin, as is done in major surgeries for example. A quick wipe with an alcohol swab greatly reduces the number of bacteria on that portion of the skin but is unlikely to bring the number to zero. The needle probably usually picks up some bacteria, I would think. I don't know of research in that area, though.
This is not a problem because:
1) Most bacteria are not pathogenic
2) Generally speaking what is required to start an infection is a bacterial load beyond some given amount. A single bacterium, or ten of them, or what have you does not start infections except in immune-compromised individuals.
This isn't to say that it is acceptable to be nasty in how the injections are done, but there need not be the fear that a single bacterium will cause a problem, either.