Perhaps the HD employee was suggesting another product, but this version from Rust-Oleum specifically says "Do not use in areas directly exposed to flames (e.g., inside of grills or fireplaces) ."

High Heat Enamel

Could the fireplace have been painted inside, so it looked nicer, because it is non working or unsafe to use? If thats not the case, then you will need to use a stripper. High heat enamel cannot be used on the inside. At least all the high heat enamel, I have used, has had that right on the can.

Thanks both for your input. Looks like I'll be returning this.

Any thoughts as how to take the paint off? Thanks!

Fire?

No stripper. It needs to be shell blasted. Stripper only liquifies it and allows it to go deeper into the porous firebrick. It must be mechanically removed.