I don't see what change they can make, at least to an iPhone. The glue is necessary for water resistance.
So why can't I buy the glue?
If it is a special glue that needs to be heated (or something), I should be able to make/buy an oven the does the cure procedures.
Glue is not required. Gaskets and other methods exist.
Necessary? Gaskets and o-rings haven't been invented yet?
There were models that were both waterproof and not glued (the only tools needed for a battery swap were the replacement battery and opposable thumbs). I never had/tested one myself though, this is just going off of the manufacturer's claims and IP (ingress protection) certification