enough luxs?

Of course, we would only know for sure if we know the Celts' lux situation, respectively how that deal looked like 20 turns ago (IF it existed at all then, which I assumed). I guess it wasn't a new proposal, but a re-negotiation.
At least such kinds of strange 'offers' often occur when deal re-negotiation is checked in prefs, the deal only contained resource deliveries (plus possibly gpt, but no upfront sum) on either sides and the player gets native access to the resource before the deal exceeds the 20-turn limit.
The most weirdest 'offers' could even come w/o any stuff on the AI's side. This is the case if the old deal didn't contain gpt on their side, but only the resource. Happened to me when I got coal from Egypt and I paid a lot gpt
and some resources. After 20 turns, I got coal on my own. Crafty Cleo surely wanted to renew the deal, but her side of the table was just plain empty...

Therefor, it rather *looked* like a really insulting tribut demand, but the 'counter proposal' option helped to clearly differentiate here.
Anyways, the AI is of course famous for making funny proposals in general. Everybody probably knows offers like WM for WM plus Fission (@monopol price)...