The only thing I can think of is the ones letting them through thought they were friendlies. After all, Wagner has been fighting alongside the Russiand for a while now and they were waving the Russian flag and wearing Russia patches.
So...idk how you decide who was acting against Putin and who just didn't know what was going on.
If you were say 32 blokes manning a checkpoint and 25 to 35 thousand of the most battle hardened troops Russia could field came through would you really leave the barrier down and say no you can't come through.