Seems kinda weird…
You’re talking about the Mega evolution for Steelix correct? Can’t find a “Fire Type Steelix” delta on the wiki list of deltas.
As for why it can’t learn any fire-type moves, is because it’s ability Foundry just turns all it’s rock-type moves(Stealth Rocks included) into Fire-Type, and then giving them a +30% damage boost, while also allowing them to get STAB from Steelix becoming a Fire-type. Not to mention; if it knew say, Flamethrower or Flame Charge, and it was just regular old Steelix, that’d be weird to see =/
Steelix doesn’t learn fire moves,its mega I enables it to turn rock move into fire type and gives a stab+30% boost. Simply:Mega Steelix I uses a rock move;Turns into a FIRE move.