Time Raveler + Spell Queller

Teferi, Time Raveler static ability says that your opponent may only cast spells at sorcery speed
Let's say that you have a Teferi, Time Raveler and a Spell Queller who has exiled an opponent's card (any card, but let's say Tarmogoyf for the sake of argument.)

On your opponent's turn they cast lightning bolt and kill the Spell Queller; Queller's ability goes on the stack. The ability; while it's resolving on the stack offers the option for the opponent to cast the spell; however, there is an ability on the stack (currently resolving.)

And there is the crux of my question; is the ability that is allowing you to cast the spell causing Tarmogoyf to technically be cast at instant-speed or does Teferi's static ability interact with that and cause your opponent to be unable to choose to cast the Tarmogoyf?

If they ever remove the Spell Queller while you control the Teferi, they cannot cast the spell, regardless of what sort of spell it is, what part of a turn it is, or who's turn it is.

This is for the exact reason you state, that the spell is cast "as an instant" because the Spell Queller ability is on the stack.