Well, as more or less confirmed by following chapters, it is untrue.
How do the following chapters confirm that Yuu thinks he no longer loves Hikari? Even in this chapter, Yuu is saying that he's doing all of this for Hikari. Whether that's true or not is not the point, the point is what the characters believe is their motivation at the given point in time.
Not really, and we know from Yuu's POV chapters how it went.
Yuu says Aya has always known that he's loved Hikari, which indicates that Yuu's love for Hikari was continuous. Yuu's POV chapters also don't say anything about his love for Hikari fading away - we get the start and end of his relationship and a morotorium on love that Hikari knocked down w/ a feather.
ETA: Yuu's POV talks about finding love with Aya, but in Aya's POV he doesn't tell her except kinda maybe responding to her prompt when they're breaking up (Aya's so head over heels that it would have absolutely shown up in her POV if he had) and here in the heavily qualified he can't imagine a reason he'd do horrible things to Hikari except out of love for Aya. So in Aya's POV, his feelings for her are at best a shallow floaty thing, the impermanence of a summer fling, in comparison to his feelings for Hikari.
Basically you shouldn't take the translation too literally, because what this says
The Russian translation goes w/ "you love another/you've known that from the beginning" which is the same vibe, and Google translate gives
"But Yuu, you're in love with someone else—Hikari—aren't you?"
"You knew that from the moment we first met, didn't you?"
so I think you're gonna have to translate here.
but even you'd admit that a lot of stuff she said there wasn't something she actually meant.
We went back and forth on this but no, I generally ascribe to Hikari's take that Aya is mean but honest. Like if she really didn't feel the thing than she'd just accept Yuu. ETA which yeah sure maybe we're getting a Yuu/Aya ending where she takes it back 🤷♀️
Which here "I said something I didn't mean again" is aimed at Aya fans (and presumably Aya/Yuu shippers) and is generally about her rejecting Yuu.