I just did a quick measurement of some samples of PVC I have laying next to me.

EX-CEL FF = 3.5"x4.75"x.75" = .250lbs = 49.875 in^3/lb
Komacel = 2.5"x6.0"x.75" = .245lbs = 45.92 in^3/lb

So as you can see, the EX-CEL FF is actually more dense than the Komacel, which is designed for exterior usage. We haven't seen the sign you're making so it's impossible to truly guide you but my guess is you'll be fine using the material they've provided so long as you keep in mind PVC moves and this PVC might expand a bit more than you originally figured.

My personal opinion is I'd stop where you are. Demand a refund. Get an exterior grade product. Why? Because you've just started the project and why take the risk? You're going to dump hours of labor into this project and hundreds of more dollars of materials. Again, why take the risk? I have a hard time believing that you can't get any exterior grade PVC or MDO within 2 hours of you. MDO is relatively common stuff for a lumber yard and even if they don't stock it, they should be able to get it from their supplier as part of their next shipment. I can't imagine a plywood supplier not having a pallet of MDO sitting in their warehouse. It may come unprimed, but many people prefer that as they don't care for the pre-primed boards. To put things in perspective how common MDO should be, Harbor Sales, a "local" sign supplier, has hundreds of sheets of MDO available in different thicknesses and sizes and they focus on selling to the sign industry.