The idea behind the rigid bathroom fan duct is that the flexible type tends to collect condensation which results in duct sagging and accumulation of condensate in the low spots. There are also BPI Standards that require air ducts running through unconditioned spaces to be insulated.
There are also exhaust fan manufacturers recommendations (the following is from Broan):
What type of duct is recommended, rigid or flexible?
It is recommended, where possible, to use rigid duct. It has less resistance to air flow and allows the fan to operate much more efficiently. If flexible duct is used, be sure the duct is as straight as possible.
You may need to call your local building department for advice or convince the inspector that there will be no condensation since this property has no heating system and it is only used on seasonal basis / during warm months.