Was this their first vacation rental?  Might it be a start-up expense, to be amortized?  Or added to the basis for depreciation, if the sale closed anyway?  Do they really want to claim it all in 2021 even if they could?  

Was there a loan application involved, where they disclosed the property was to be used as a rental?  I would want to know a lot more facts and circumstances before venturing a guess.  But I'm sure if you asked three people at IRS, you would get two different answers and an "I don't know."