Really, nowadays, with the rubber available, 3mm arms are no problem. Was I a wetsuit maker today, that might be the suit I'd make. That said, I think that rule sounds dumb.

Pussies!
That's my opinion. 68 F is freakin' warm here in SoCal and many other places.
IMHO FINA has ruined many traditions ( my traditions being limited to California beach starts and finishes) of open water swimming with their dock starts and touch pad finishes.

For a swim only event, I would put the wetsuit limit at 60 or below, but even that should be a separate pussy division with the medals going to the non wetsuite swimmers.