Ask Lemmygrad

In terms of genealogy the Finns are most closely related to neighboring Finnic groups Estonians and Sámi, not Russians specifically, East Slavs in general, or any other Slavic population.

In terms of culture Finnish culture is largely indigenous with a mix of a larger Swedish influence and much smaller Russian influence but is still culturally Finnic in most respects.

In terms of language the Finns speak a Finno-Ugric language like neighboring Estonians and Sámi, not a Slavic language like Russian or anything else.

Politically speaking Finland has long gravitated toward Scandinavia and thus the West and less so Russia or other Slavic countries even when it was part of the Russian Empire.

Religiously Finns are mostly Lutherans, itself an import from Sweden, and there is little Orthodox presence to tie them to Russia's own religious history. They also have an indigenous religion that is very clearly Finnic, not Slavic, in its customs.

So in every possible way that could matter: no, Finland isn't Slavic regardless of how the word is being used.