The beginning of a slur will have a different 'attack' from a note anywhere else in that slur. It's, as you say, the tonguing of that note. The others are simply blown, in the same breath, so there's no attack apeart from the start note.
Often, the music itself will tell the player how to slur, but that's maybe a different interpretation from that of the composer. So, that composer needs to envisage how phrases are played, and phrase, or slur, accordingly.
What better reason to at least have a foray into flute playing? It's the same for any instrument that's being composed for - each has its own foibles - subtle ways to produce sounds - and without knowledge of those, the composer is the poorer for it.
Leaving out slurs will mean the flautist will inevitably find where to put their own in, which may, or may not align with the composer's ideas.