All fiberglass boats, unless modern (in say last 10 years), have wood fully encapsulated with fiberglass as transoms. Some may have foam stringers but they are few and far between - rare as hen's teeth if you will.
Or to put it another way...you got a screaming deal on the boat because the previous owner knew something that wasn't shared at time of purchase. Probably getting rid of a headache or two and priced to move so they didn't have to deal with it.
I'd proceed with as thorough an inspection as practical to determine the actual integrity of the transom. You might have lucked out and found the true unicorn in the bunch but going through the restoration forums, that's not too likely.
Now...if you happen to find out that this transom is shot as well, might I suggest a pourable product such as SeaCast? Leave the inner/outer fiberglass in place and clear out all the wood inbetween. Then pour this in and let it set. That will net you a composite transom that won't rot again. It won't be cheap but it doesn't require a ton of grinding IF, and I say IF, the transom is the only issue.