Hironobu Yamashita kindly tells me that this comes from Omega and is documented there, also that e-ptex has fi glue as well but in that case they keep \gluestretchorder compatible with e-tex.
https://chat.stackexchange.com/transcript/message/51983994#51983994 So I suppose this is a request for the \gluestretchorder difference to be documented in the manual since the only mention of \gluestretchorder in the luatex manual is in the list of etex commands. David
