Or he doesn’t consider a source found in late Achronim to matter when no earlier sources mentioned it first.
But why do you think the Rogatchaver Gaon would say or quote it?! And why do you think the ????”? himself would say it, if not for it being a reputable statement?!
Also, everyone should agree that just because something is found in Seforim, especially the Arizal (like the topic of our discussion), without earlier sources does not mean it isn’t considered a source. See the end of the ????? of ?’ ???? ????? to ??? ??????? starting with ‘???? ??? ??? ??? ????’ until ‘?????? ???’.
Some examples he brings are ????? ????”? ?? ????? ???? ?”? and on ????? ??? ?????? ?”?, see there.