Reiner doesn't blame Hakkai for stepping back. Between the Armored's fearsome appearance and the violence he just inflicted on those kelpies, it's entirely understandable. Reiner remains where he is, not dropping his hand for Hakkai to climb onto his palm. That will come once Hakkai is reassured—if it happens at all.

"Yeah, I am," Reiner answers honestly, not bothering to explain the one exception.

The very first time, he wasn't in control; no one ever is. (Unless their name is Bertholdt Hoover, but Bertholdt's incredible aptitude was the exception, not the rule.)

"I inherited my Titan when I was ten. I'm eighteen now. I've used this power more times than I can count. I have complete control."