I remember this, and I also remember the trapinch sets (don't remember Diglett ever being used). If just trapinch got arena trap and dugtio didn't, I highly doubt it would be even suspected. The sets were annoying, but on the level of magnezone trapping your steel types. I guess I was and still am a pretty bad player, so it's possible I am wrong about that. But like, would we say that arena trap is less deserving of a ban if feebas got it as a hidden ability?
arena trap has the same dynamics in-battle regardless of the pokémon. it was determined that arena trap in & of itself was uncompetitive, as it has no real counterplay. diglett was used as it eliminated key threats to stall such as heatran, cb tyranitar, & others. the baby goths also did similar things with trick scarf & other choiced sets.
personally my stance back in the day (& to some extent now) was that all trapping is uncompetitive, but others argued that because of magnet pull’s far more limited scope, it wasn’t worthy of being in the same category as arena trap/shadow tag, & that stance won out in the end.