I mean I think the gap between Moses and Mekari is very small. Miss very little if Moses misses time and Mekari fills in.
I think Mekari is getting extended to reduce his cap #, considering he's 5 years younger and can play five positions on the Oline. He was the reserve Center all last year basically, since they never had a reserve Center on the active 53 man.
There's also no certainty that a) there's going to be a draft pick and b) that draft pick will be the Swing at LT in year 1. Most likely, Mekari will remain the swing at LT, and it'll be Faalele and a draft pick competing at RT.