Hi Willie,
very good that you accepted - and followed - this Strida advice!
I believe the main reason for this restriction is the simple reason that the rear hinge and the locking mechanism are too weak to withstand an overstressing (by torsion) created by stronger riders plus standup pedaling.
In your case it can’t be wrong to inspect both bolts (Strida part number 274) left and right of the locking mechanism frequently for tightness. Please do not attempt to ride the bike with a loose bolt - never.
I’m missing the frame clip at your bike…
Note that - if you secure the frame tubes in folded position by “crossing” them - the belt tension will increase.
It is not recommendable to overtension the belt; therefore I’d choose belt tension (in unfolded position) just a little bit lower as normal 
Moreover that you are using an aluminium beltwheel - which I do like very much 
- but these don’t flex (as the original plastic ones); at least therefore I’d keep an eye on this
But anyway I’d recommend the frame clip - it will help unburdening the rear joint.
stridacanada.ca/product/strida-frame-clip/
Thank you,
Chris