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This was done with the +10 Summoning boost from a Clan Citadel battlefield. (Thanks to Isobel for allowing access to the battlefield)
Not sure if this is u/JagexFrosch's hidden dialogue or not, but it wasn't previously on the wiki and the baby dragon pet page stated that there was no way to understand baby dragons using this boost.
Ah! How did I not see this earlier, my notifications must have been bugged or something! Yes, this is the thing I was talking about ^^ I noticed that baby dragons were never given their own dialogue when I moved them to the pet interface, despite the fact that if you boosted your summoning at the clan citadel battlefield, you should be able to understand them going by the established rules. So I wrote some dialogue :D
Congrats for finding it!