Yes, it will work, any previously supported AMD card from the HD 7000 series and up will support Metal and therefore Mojave.
Do you try to install public beta macOS Mojave with R9 280X GPU?
I trying today on 2 machines:
iMac (Retina 5K, 27-inch, Late 2015) with R9 M390 2GB - and installed successfully
Mac Pro 5.1 mid 2010, 2 x Xeon 5680 3.33GHz, 64GB RAM, R9 280X 3GB - and I can't install Mojave
Only get info:
"Installing macOS Mojave on this Mac requires that all graphic cards have Metal support and that FileVault is disabled"
Double checked FileVault settings on my MP and is disabled, R9 280X is fully compatible with Metal... but the problem remained
So, for my opinion Apple don't care about supporting all "Meta;" compatible GPU's just trying to force us to buy some expensive "recommended by Apple " GPU's like external RX 580 for $599 ;-)