Elephant archer has aready 100% accuracy even without thumbring. Thumb ring would only make them attack a bit faster. So their elephant archers wouldnt be as trash as their cavalry archers
The lack of 2 armor upgrades can be somehow mitigated by howdah even in castle age.
I have a low elo match recorded online in which ai defeat a mangudai army with elephant archers in arabia. My oponent and I just decided to mass UU st the same time and their mangudai were wet paper against my elephants. Had he raided me, the resould would have been different, but he panicked and tried to stop my push