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In Japan, you don't give a kiss to someone casually. Especially a first kiss. A sacred first kiss.
The sacred first kiss is considered to be an "adult" kiss reserved for someone you truly love and is a rite of passage for many Japanese youths becoming adults. The kiss is so intimate that often, it has to be done in private, far from public eye. If such a kiss occurred in a children's show, it must be off-screen. That's why the anime can never show Serena actually kissing Ash on the lips. Heck, that's why you don't see Professor Kukui and Burnet kiss, even during their wedding.
Also, the sacred first kiss is something to cherish and remembered for the rest of one's life. And it does require the other participant to respond by either rejecting or recuperating it. A lack of reaction is considered rejection if Shinji and Asuka's first kiss in Neon Genesis Evangelion was anything to go by (Namely a miscommunication plus toxic interactions causes Shinji to be confused by Asuka's kiss and thus he didn't react out of fear, and Asuka ends up believing that Shinji's non-reaction means he's rejecting her when in fact he's not).
Thus, based on Ash's reaction after Serena's kiss, we can infer that it is one of acceptance because his eyes twinkle and his surprised face turns into a smile before he wishes Serena luck. Combine with Bonnie giving Serena a secret thumbs up behind Ash and Ash reminiscing his time in Kalos, including his time with Serena, it's hard to claim that Ash did anything but recuperate Serena's kiss, however subtle it may be. Now, Yajima states they are not yet a couple but could be in the future is an acknowledgement that, officially, Ash is still "10 years old" and thus would be considered too young to be pursuing a serious relationship upgrade right now by technicality. However, by stating the two would be a couple in the future, it ensures a romantic closure for their ten-year old selves that would be only be reopened when they meet again at an older age.
