And casts them into a definitively romantic light rather than the ambiguously romantic light we (and Tsukushi) have been seeing up until now.
Pretty sure most of us have had no doubt whatsoever from the start that it was mutual.
That may be so and I agree but before there was at the very least some plausible deniability there. Now it's 100% undeniably crystal clear xD
yes, because like while they were already clearly mutually romantically interested in each other it's only now that we see things from Hinoka's perspective that there is no situation in which their feelings are somehow not mutual. Not that I believe that the author would pull such a stunt but the earlier chapters and Tsukushi's behavior are all based on the idea that Tsukushi doesn't know for certain and is scared of being rejected like she was in the dream that kickstarted all of this.
And so it is only at this point, chapter 14 that we the readers finally get more information than Tsukushi has. Which is that one hundred percent for a fact their feelings are mutual. And Hinoka is one hundred percent for a fact aware of her own feelings and not, like Tsukushi was for a long time, in love without realizing it.
Which tells us a lot about Hinoka's reactions to Tsukushi budding flirting and why she reacts the way she does when Tsukushi ends up asking Hinoka to please wait a little longer because she can't tell her what she wants to say just yet. Like while to us it was never a question whether their feeling were mutual it was a question whether Hinoka realized she was being flirted with, or whether she was actively flirting back. But now we know that all of Hinoka's actions and reactions are because she's actively trying to build up to a romantic relationship.