He should have it, but it didn´t kicked out, he never had his exoskeleton awakenend.
Reread it bro, he didn´t have it back then.
Means so far Sanji could improve by his "OWN" without getting his exoskeleton awakening.
That was the difference to his siblings...
I dont think its something that wasnt there at all and suddenly kicked out all at a go
nami saying his body is a bit harder than normal is classic foreshadowing is why so many of us were convinced exoskeleton might be a thing later down the road
how durable he has been up until now is very likely a result of his dormant genetic modification.
no case of humans in one piece improving their "durability "
you cant improve how hard your bones are......