"De" is agreeing with "vise(s)", which is a plural in this case, rather than with "stein", which is indeed a singular. We're not talking of just "en stein", because it's a specific stone; the stone of the wise.
Literal translation from the Norwegian original:
"De vises stein"
"The stone of the (plural) wise (ones/people/men)."
Literal translation had the Norwegian been referring to a singular wise person:
"Den vises stein"
"The stone of the (singular) wise (one/person)"
Literal translation from the English original:
"The philosopher's stone"
It's not a literal translation, but rather the names used in both languages for the same thing.
Note that Wikipedia (https://en.m.wikipedia.org/wiki/Philosopher%27s_stone) also thinks that philosophers' stone would be more accurate (the Latin they give, presumably the original name, definitely uses the plural).