Not true. There is a free Infocom interpreter for the Commodore Amiga,
so you can play Inform games on that.
Infocom released some version 3 games for the Commodore 64 (at least
Zork I, Zork II, Zork III, Deadline, Starcross and Suspended), so if you
can get hold of one of them, you can probably patch the interpreter so
that it will play Inform's .z3 story-files (for example, Release 10 of
"Curses").
I don't know if Infocom ever wrote a version 5 interpreter for the
Commodore 128. If they did, then you will be able to patch it so that
you can play Inform's .z5 story files.
-- Gareth Rees