When a Phantom Knight's Trap is Special Summoned back as a monster, and then is sent to the GY, is it still treated as a monster?
From: Trap Monster - Fandom.com

...and can in some cases be used offensively as well. Also, because they don't count as monsters while in a player's Deck or Graveyard, some effects might work on them as a Trap Card- ...

A trap monster is only treated as a monster whilst it occupys a monster zone. When it's sent to the graveyard it is reverted back to being treated as a trap.