There is no reason why there should be any difference between folded and unfolded:
- the freewheel is fixed on the lower frame tube (actually on the axis linking seat tube and lower tube), so its relative position to the chainring does not vary.
- the lower tube defines the rigidity of the transmission and attaching its end to the steering tube does not affect it.
According to me, nothing should prevent you to adust the belt tension on a folded Strida, it is even more convenient than when it’s folded!