As someone with thousands of hours across Dota 1&2 I will say the moment to moment gameplay in Deadlock is completely different. It scratches a TF2-type itch, in addition to MOBA gameplay. I think what you’re right about is that the target audience might be smaller, in that you need to be good at both MOBAs and shooters to be good at this game. But from what I’ve seen of the big streamers adopting it, learning the MOBA parts to a point of “good enough” won’t take that long.
In fact, counterintuitively I think it’s the MOBA playes migrating over who will struggle. Aim and movement is extremely crucial, and if you can’t aim you will suck at Deadlock regardless of how many Dota tourneys you’ve won.