>>39429861They didn't need to or they overlooked it. While not biologically correct it still could be taken to mean Mew gave birth by laying an egg without needing to add anymore dialogue.
Either way its a singular line of dialogue in a 22 year old game that is no longer relevant in the advent of Gen II and onward where breeding mechanics were finally introduced and built upon over twenty years. Mew giving birth was never referenced again even to clarify whether Mews lay eggs or give live birth. Since all pokemon who reproduce lay eggs, even the weird ones like Ditto (who is a botched clone of mew), its a pretty safe bet that Mews lays eggs too as Mew is a Pokemon.
If Mew reproduces it lays eggs, end of story. Unless you can give something other one out of context sentence written by a nutter in Gen 1.