We want Lassila to be the new drummer.

[Jabi]

You are correct. In some places the credits may be displayed incoherently, iow. like this:

Future Shock (Tolkki-Lassila)

but in the album notes it says all music was written by Tolkki.

Anyway, Antti Ikonen has been composing music throughout his career, mostly music for theater plays. It's quite likely that even if he didn't receive credit, he wrote (at least some, if not all) his keyboard parts himself, thereby influencing the overall sound of the first albums.