It’s a big investment and expansion but I don’t think it’s a hub. More like a test balloon.
I’m not close to this one but Delta did something very similar in MIA back in the early teens. I’m speculating at this point so if you’ve got better facts let me know, genuinely. I’m a relative insider but not close to network strategy or CRE so this is mildly informed speculation.
First off, both Delta and United would love to have a hub that far south and east. Delta has ATL and United just has IAD and IAH. IAD needs relief for sure.
Set aside trying to horn in on MIA (not totally infeasible as AA is weaker - especially on their international fleet if memory serves), the airport that makes the most sense by far is MCO - especially for United. But high rents and local costs are probably prohibitive.
FLL is motivated to attract new tenants so it’s very likely that United has a deal with something like no landing fees for the first year - which is how Delta operated an LHR flight out of MIA for a year as a comparatively low cost experiment.
This was all human generated by my faulty memory and very limited expertise in this area of aviation - so take it all with a grain of salt.