What the picture indicates is that the 4 hits during the attack round from Hermaeus totaled 100,864 damage, not that he did 100,864 damage in a single hit. This is why the number is higher than the 99,999 cap.
I'd be willing to bet that whatever mods you're using to drastically change your chat display is able to read and output the actual damage and bypass the official 99,999 display cap.