Well, there have been traditionally multiple types of love.
Philos--familial or brotherly
Agape--universal love, given without regard for one's relationship to another
I don't think the panels around Gramps or Itsuki's comments on the matter necessarily imply romantic love (see pic related, for example). It would be weird to hear a grandpa talking about using romantic love to differentiate his granddaughter, and likewise for Itsuki using that to differentiate her sisters.
The way those conversations played out, especially with gramps in 66, seemed to suggest Fuutarou will have to learn to love all of them to pick them out. We do know, though, that one out of all 5 will be particularly special and he will marry her.
Feeling romantic love for someone basically implies he or she is special. If we take Miku anons seriously that Fuu loved Miku at or before 67, then she's basically done in the bowl. Chapter 68 told us it was the bell kiss, and the day it happened, where he began to think of the winner as special.
Ever since 68, we haven't been shown anything in Fuu's relationship with Miku that really show that she's special. Negi's spent this whole series showing us the info necessary to reach and understand the eventual conclusion, so him keeping Fuu's interactions with her so muted and boring, especially on his end, would be odd if she's the bride.
We do know, however, that Ichika and Nino have been on his mind.
And since that hug scene was referenced when Fuutarou mentioned how he figured out her feelings, we could conclude that he did indeed use love to figure them out. Specifically, he used her love for him, not vice versa.
And Miku already mentioned, possibly foreshadowing, during the camping trip that she figured out that she wasn't special, at that point, at least.
This isn't that hard. It's not the Voynich Codex. It's a shonen manga.