Hashirama didn't recognise Saru, and the last we saw of them together Saru was still a kid.
Tobirama was Hokage long enough to get his face carved on the mountain, set up the police force (and have a prison and police station built), lead his country to war, travel to the cloud to sign a peace agreement, recover from his injuries, return to Konoha, continue training Saru, and appoint Saru Hokage when he looks to be (at least) slightly older than Naruto is now (late teens probably). Saru then goes from being a teen, to being an adult with a goatee (looking significantly older), and has since had his (bearded face) carved onto the mountain. This Saru trains Tsunade from age 6 as far as we know (since that's when the Sannin graduated from the academy). It's more than a tight fit... Tsunade looks at least 3-4 in that image of Hashi remembering her... it's impossible to fit all that stuff in that timeline.