This is valid, but:
1) As mentioned before, X is more tolerant of niggerword posting now
2) Even before the Gab influx, there's a natural bleed away from Poast towards places with a more defined culture (I headed to baest *rapidly* after seeing it, and I think this type of behavior is typical)
3) Graf himself encourages people off his instance. He has multiple valid motives for doing so.
I don't think pinning it on one cause is right.