SMU sat out 1987 and 1988, then resumed play in 1989. You can't really "effectively" be a 7 team conference if you have 8 teams, and regardless the article fails to even make the distinction.
Actually the Southwest Conference prior to 1990 was Arkansas plus EIGHT schools from Texas. They were Texas, Texas A&M, Texas Tech, Baylor, Rice, SMU, TCU, and Houston. The league existed as an 8-team conference for 4 or 5 years after Arkansas left.