Even seeing the X, and knowing that work was in progress, it’s hard to wrap your head around the notion that you should land on the very narrow taxiway near where planes are tied down/parked.
Well pretty sure there's some legal aspect to landing on surfaces not surveyed/approved for the purpose.
The major issue you need to consider is that even if the runway is closed there are various things that taxiways could be used for, many engineering purposes could see aircraft doing ground runs, taxi trials, taxi checkouts for maintenance staff. None of which require a NOTAM issue either as they are just daily operations. The runway closed could signal they have the taxiways to themselves to get some stuff done in peace and quiet without having to get out of the way of traffic, don't assume runway closed means airport totally deserted.