That would have only delayed arrival of the delta variant by a few weeks at most. The US has always had de facto open borders with no real effective security.

Delta and other variants are not unlikely events though: the expectation is that they would emerge independently anywhere that the virus is freely replicating.

Delta is so named because using "origin of identification" kept convincing people the virus came from there, rather then was first isolated in a lab - and that relatively straight forward mutations couldn't emerge in multiple places with widespread infection simultaneously.