When her getting mad was literally the last thing in the chapter? His time to speak on the matter comes next chapter. It wasn't anywhere before this.
People getting mad that Yuu didn't immediately confess his love for Hikari or confess to Hikari about Yami are ignoring one of the key components of Yuu. He hesitates constantly. He doesn't commit to something crucial unless he feels he's at or near 100%.
Look at past examples. He mentions he didn't try hard enough to get into Hikari's high school and he desperately regrets it. After he's ghosted by Yami and he's hanging out with Hikari he never presses the issue or confesses to her as he's not 100% sure she feels the same way.
Based on the information he has, he has no reason to believe Yami and Hikari are friends let alone acquaintances. He's not going to reveal any of the Yami stuff as that would lead to Hikari's acceptance of his confession (if it ever comes) to be less than a 100% chance.
Look back to when Yuu finally catches up to Hikari after the Yami slap/kiss. Yuu knows Hikari is angry but assumes it's because he ditched her. He thinks there's no way she would know about the kiss or anything else about Yami. So he doesn't say anything because that would lower the chances of Hikari liking him.
Later in the storage room when he's talking to Hikari he finally feels he's at or near 100% and confesses to her. Hikari calls him out for not showing anything until now and Yuu mentions he hesitated because he wasn't sure Hikari felt the same way. He rightly gets called out by Hikari saying did he really not notice all the effort she was putting out. Yuu mentions he noticed it but wasn't sure it was affection. He even literally says "That's why I've been treading carefully, looking for the right timing...I was thinking about taking my shots only when I'm 100% sure."
What changed? What caused Yuu to feel that now he's 100% sure (or close to 100%) Hikari likes him?
I have a hard time believing Yuu was originally planning to confess to Hikari during this festival. I'm near 100% certain he decided to confess based on whatever transpired between him and Yami during the classroom scene. I feel the chances are good that the conversation between Yami and Yuu was something along the lines of Yami perhaps apologizing for ghosting Yuu, telling Yuu that Hikari is her best friend now and she knows Hikari was/is the one Yuu likes and that, most importantly, Hikari likes him back. Then, knowing Yuu, he hesitated again which caused Yami to get pissed at him and slap him. The kiss was more of a "farewell to what we had" thing than an "I still love you" thing. Or maybe it was a "I still hold affection for you but you need to move on and go to Hikari" thing. Either way, it seems more in Yami's character to be self-sacrificing, especially even more so now that she hold such affection for Hikari.
Next, we get the start of the real issue which is that Hikari knows about Yuu and Yami and is mad that Yuu is confessing his love while less than 30 minutes ago he was kissing her best friend.
Yuu mentions he feels he had to confess today. In the novelization Hikari asks him why today and he doesn't say anything. Hikari has an internal monologue (also I think the first line is mistranslated and should say "I desperately" not "he desperately" as keeping it as "he" implies it's Yuu who's trying to hold back from saying anything when it's really Hikari who's trying not to blurt out she knows about the kiss).
For just that moment, he desperately tries to hold back the mouth that has been slipping all this time.
I put a lot of pressure on my lips, holding everything back.
One second, two seconds, three seconds...
Then, with the same force, lift the corners of your mouth as much as you can.
Hikari knows Yuu's full of shit but she's trying to give him a chance to come clean. Obviously, Yuu has no clue until Hikari pushes him to the ground. Then we get this:
He was still lying on the ground, looking up at me.
His gaze, his expression, told a story.
...He said he had an idea as to why this was happening to him.
Finally, Yuu is starting to realize that Hikari knows more than he thought she did. And he has an internal monologue that says this:
I see, I thought so...
I had noticed something was different about Yokari.
They were watching me.
They knew about "that"...
I'm not lying.
I've loved you for a long time now.
But there's no doubt that I was the worst today, right...?
He's finally at 100% realization that Hikari knows about Yami and since he's at 100% he's most likely finally ready to tell the truth which is hopefully what we get next chapter.