Serena’s flight was “Latias H71" (it is said right before the kiss scene starts). Serena was going to Hoenn, so the #71 in the Hoenn dex is Mawile - or in Japanese, Kuchitto. Kuchi (Mouth) and To (with). Or in a rough translation, “Latias - but with the mouth”.

It’s the writers saying: “We can’t actually show you what they did because of censors, but she basically did what Latias did in the 5th film, but with the mouth.”