The way I read it, the whole thing was found first. But the finder only gave the 'smaller' half to some museum for study. Then, 20 years later, he produces the more 'complete' half.
As far as 'faked', the tail shown on the 'smaller' half is FAKE, in that it was taken from some other vertebrate. I guess we can call that 'reconstructed'.
However, I would agree that whatever reconstruction was done, was done in good faith.
Why did this person keep the 'bigger' half for 20 years, hidden from the scientific world?
Was it? I hadn't read that. I thought they'd probably looked at what they had, figured this animal must have had a tail, and made one that was consistent with what they knew of similar animals.