What Tatsua's saying is that a Servant needs some historical/mythological grounding, they can't be entirely OCs. EMIYA was eligible because of Counter Guardian shenanigans, Rin's pendant, and things of that nature. Fake Assassin has a historical basis but never actually existed, so the Grail searched through the Akashic Record for a wraith most similar to the legend of Sasaki Kojiro and summoned that wraith, which only ever happened because he didn't have a real summoning. The point is that both EMIYA and Not!Kojiro had justifications for being summoned as Servants, completely "original" Servants don't work in the same way. They need to have existed historically or in legend, rather than being some legend you made up.

Also Mjolnir doesn't work as his NP for a couple other reasons, but that comes secondary.