I don't think they're retconning the kiss out, the kiss still happened.
The thing is, there is no satisfactory way to address the kiss that makes sense for the characters at their current stage. Ash didn't initiate the kiss, and I'm not even sure he knows the meaning of being kissed by someone. There's no reason for him to say "oh hey Serena, remember the last time you kissed me?" because that's probably not even on his mind. We know Ash's priorities and thoughts lie elsewhere (food, battles and Pokemon).
Similarly, Serena isn't going to be like "hey remember that time I kissed you haha?", because the scene would once again lead nowhere. Either Serena would be too shy to bring it up to Ash, or Ash just would just be like "kiss? huh?", since he's clueless about romance, which wouldn't lead to any progress either, just an awkward scene.
Something could be done in the future if the characters are older and Ash is more aware of romance, but not at this stage.
So yeah, it makes total sense that the kiss wasn't brought up at all.
Maybe they could have had Serena blush all embarrassingly around Ash as a reference to the time she kissed him, but I'm kinda happy they didn't, because it once again highlights that she's more mature than she used to be.