Why didn't the Son'a just tell the truth to the UFP in the first place?
Further, there's so much to this movie that doesn't make any sense.
Apparently the only thing the Sona held back on, is that the Baku and the Sona are the same people.
Tell them there's a fountain of youth planet
It is pretty obvious they did tell the Federation Council this. Why else would the Council agree to the harvesting of the particles, from around a Federation planet?
in UFP space
Which is why the Sona (likely) approached the Federation Council in the first place. The Federation would have realized that the planet belonged to them as soon as they found out the co-ordinates.
that the Son'a have equal claim to ...
In other words zero claim, which is just as much claim as the Baku had to the planet.
and that the inhabitants aren't primitives.
The Federation were always aware that the Baku migrated there from somewhere else. The only
person in the movie who as unaware of this initially was Picard.
... but when the where they ASKED about it?
Exactly what should the Baku have been asked about?
Remember ... IT ... ISN'T ... THE ... BAKU'S ... PLANET