Good question!  I don't really know.  My guess is it's probably not
'optimal' due to a slightly different fat content but it may be 'acceptable'
if it's an adult cat and just being used as a treat.  That seems safe enough
for a treat.

Cats do not need milk after they have been weaned.  They should be
drinking only water.

This is something that I thought too a few months ago, so I gave it a go.
It worked fine, no upset tummies.  Mine two have it as a treat.