I found an faq that says the Thief class at high levels has a high chance of using Mage scrolls in the Pools series, but it doesn't really specify what level or if you need a specific attribute to be high nor does the game manual. Anyone know anything about this? Thanks in advance.

And I don't need a high attribute like INT for example?

No.

Cool, thanks. Strange this isn't mentioned in the manual.

A did some testing, just in case, and it looks like there may be an INT restriction after all. I have a lvl 10 Thief with 14 INT that can's use magic scrolls, but a lvl 11 Thief with 17 INT can. So it looks like it may be a 15 INT requirement, after all, since I'm quite sure about the lvl 10 restriction.

Ah, okay. Looks like I have some stat rolling to do.

I did some more testing, and rolled up some lvl 14 Thieves with different INTs.
With Int 6-7 they could not use scrolls, with 8+ Int they could.

So it looks like the restriction may in fact be that the thief need to be at least lvl 11 and have at least 8 Int.
Rolling a character with an INT as low as 6 or 7 is very rare, so in practice you can ignore the INT restricion.

Out of curiosity, do Pool of Radiance and Gateway to the Savage Frontier implement this rule? (In other words, if I hack a thief's level to at least 10, can she use thief scrolls?)

Edit: Or perhaps hacked to level 11?

I'm not sure. The most annoying thing about the Gold Box games is that from game to game there's always some minor rule changes.
I did my testing with FRUA.

I took a picture of the rule in the 1st edition AD&D tabletop games. Most of the Pool of Radiance rules are trying to follow this rulebook.

It's rule #3, and I don't see any follow up or additional rules anywhere else in the Thief class to further explain the rule.