I’ve never claimed it was a perfect seal, nor does Defilers and Preservers, it is, however, a very good seal. But it’s not a total lock like in Eberron, which seems completely devoid of any and all connections to the multiverse (Eberron was made in 3e however, so could get away with it. Still weird though.)
Furthermore a pair of artifacts hardly qualifies as “a lot” of holes. Note that the Black Spine adventure was made before a lot of this was settled (as was Dragon Kings, the other main source for planar weirdness), and even then the adventure specifies that the reason the Nightmare Gate was created was because the Gith found it extremely hard to leave Athas.
Ravenloft’s chunk of Athas is a notorious continuity nightmare, like a lot of things Ravenloft.