One reason why magi might not have non-gifted lab assistants is because those assistants are typically either their apprentice or full members of the Order. Both are granted special status by the Code. Outside of House Bonisagus, one cannot just come knock at your sanctum's door and take your apprentice. But otherwise mundane lab assistants are not protected by the Code, except, maybe, by the "diminish one's magical power" clause.
This means that if your assistant one day decides to leave you, you have no recourse against it, even if he decides to go work for another magus. You might bring charges against the magus, but unless he actually influenced your assistant, you're probably out of luck. A very different case from apprentice-snatching.
Then, from a purely mechanical point of view, there is the question of who is allowed to learn Magic Theory at character creation. It takes a virtue like Failed Apprentice to allow it. Again, who else would bother learning it? I see it as a justification for having MT, rather than a requirement to be able to use it at all.
Serf's Parma: In RoP:The Divine, can someone without True Faith assist someone with Ceremonies at all?
After character creation... well you need to persuade your assistant to go ahead and learn something that he'll never be able to use on his own (here's a story for you: your assistant leaves you and joins up with a mystery in the hope of acquiring The Gift).
Now, if you want to rule out completely ungifted people from being assistants, you can limit it to people with a supernatural ability, as was mentioned in that other thread. You can be even more strict and limit it to those which have an associated ability score.
But I think that if a character has, let's say Arcane Lore or Student of Magic, then, you have potential assistant material, gifted or not. They just can't start with Magic Theory.
Finally, you can simply decide that lab assistants must be companion-level characters, thus preventing covenants to just run all their servants through "Introduction to Magic Theory".
As usual, Your Saga May Vary. I choose to disregard the "and has the Gift" provision.