Previous to my reply, they hadn't said what "sealer" they were going to use. My thought was exactly what
@Babin Farms said, cover the problem, and you're never gonna know the difference until you have to replace the keel guard. Still not sold on their idea of a "sealer", when that's really just a step away from a gelcoat patch, and will accomplish the same thing they're proposing, as well as match the boat.
My other concern was, if that "sealer" wouldn't adhere (sounds like the boat DID get wet before you discovered it), when you have to pull the keel guard to RE fix it, now you have to do the same work you should have done in the first place, and replace the "free" keel guard.
If they keep it long enough to make sure it's dry, then again, why not fix it properly, and with matching material, not just a "sealer"?
I get that it's SUPER tempting to take the free fix from "the pros", but as with everything in life, free is never free. Shortcuts almost always are shorter because they leave out something, or are a faster solution.
You can get 2 things from the list:
Fast
Good
Cheap
Take your pick, but I have yet to see all 3 of those things in any solution, and if I were in your shoes, I would take them up on the keel guard, and see if they'd split the cost of the gelcoat fix, and NOT just a sealer.
Sight unseen was a guesstimate - have them LOOK at the damage and give you a quote. None of this gets you on the water faster, but they MAY end up having your boat longer than you expect too, so... *does the hand scale gesture*. Six of one, half dozen of the other. Good luck with whatever you decide, and don't second-guess yourself, after it's done, go put some smiles on those motors