One, it wasn't a serious slash. It's a one-handed slash intended to test WB, not kill him. Two, yes, Jozu would stop and shrug off Daigunka coz it can't burn him if he's in full-body diamond form. At worst he gets pushed back because Akainu could still apply pressure, unlike Mihawk.
Even Ace - a fire Logia - could still be burned by Akainu's magma. Jozu's diamond's will be melted.