No. I worked with both SCO and ISC linux in the late 80's and
early 90's and run level 5 was used for X. In fact I think
it was used also
in DGUX for X.
I don't know about ISC UNIX (aka Interactive UNIX) but
SCO did not use run level 5 for X. I cut my teeth on System V UNIX
including SCO UNIX 3.2 and seeing X in runlevel 5 these days still feels
wrong to me all these years later, though I have to come realize how convenient
it is.