No, they can’t be.
If they’d just put on some swim goggles, they’d have already been in the US. Cubans do not go to southern Mexico to get to the US. South America and other countries we don’t want here do.
Yes Cubans absolutely do come across this border not at all uncommon, it started when the O changed the “wet foot, dry foot” policy. It makes no sense but yes Cubans do cross this border.
In fact everyone crosses this border; Chinese, Cubans,you name it they all come across. I sit here and wonder why, no answers. Makes no sense to Border Patrol either.