OK, I think I answered my own question.

I found a copy of the label online HERE and read through it more closely. Here's a blurb from it:

XL 2G may be applied as a preemergence treatment for control of annual grasses and certain broadleaf weeds in
established warm season turf including bahiagrass, bermudagrass, buffalograss, centipedegrass, St.
Augustinegrass and zoysiagrass or established tall fescue growing in warm season areas.

and

Do not apply XL 2G in the spring or early summer to tall fescue turfgrass reseeded the previous fall. In such
cases, apply Balan* 2.5G granular herbicide at 60 to 80 pounds per acre in early summer (Round 1) and XL 2G
at 100 pounds per acre approximately eight weeks later (Round 2).

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In the store, I just saw the bit about it being safe for established tttf lawns...and would've considered my lawn to be 'established'. I overseed every fall, so there really is quite a bit of newer growth. I think I'll use something else, just to be safe.