"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"