That's correct. Top 40 was never meant to be synonymous with "rock and roll." KLIF in Dallas was a Top 40 station before the rock era began.
In suburban Detroit there's also a station known as WCXI, 1160 on the AM dial, which bills itself as "rare oldies" (the format is actually on that frequency temporarily while the station that normally airs it, 1460 WPON, is off the air looking for a new transmitter site; WCXI's normal format is actually classic country). Most of it is pre-Beatles rock and roll, pop and doo-wop but they also have a rotation of pre-rock songs in there. I've heard "Wheel of Fortune" by Kay Starr, "Come On-a My House" by Rosemary Clooney and even "Be My Love" by Mario Lanza there. They stream at wpon.com.