This is explain in assembly manual of Si5351A of QRPLabs.What we do not know is just how sensitive the WSPR decoder is to frequency accuracy.
I use 26MHz TCXOs and have decoded my 2m transmissions with no problem.
There was some discussion a few months ago about using even a 10MHz reference for the Si5351 but nobody thought to try WSPR using that.
WSPR has to cope with borderline HF propagation, TX and RX drift.
Without knowing the tolerances of the WSPR decoder it is wise to keep as close to the specifications as possible but I suspect the tolerances are wider than many think.
73 Alan G4ZFQ