Turns out you're both right. Stewart and Holley were bass players and contemporaries, and they both sang while they soloed on the bass. Of course each guy had his own style.
Slam's thing was to simultaneously sing an octave above what he was bowing, a technique he started in the mid-1930's until it became his trademark style. Stewart was perhaps best known for his duo novelty act with guitarist and singer Slim Gaillard. In the late '30's "Slim and Slam" blew up on the national scene with their hit "Flat Foot Floogie." Later Slam would play with Art Tatum and Lester Young and then start his own group featuring a young pianist named Erroll Garner.
Major Holley's approach was similar to Slam's, but instead of singing an octave above, Holley sang the exact same notes he bowed. No easy feat considering there are some low bass notes. Holley, otherwise known at "Mule," started off playing tuba, but he later switched to bass when he was in the Navy. He played with Dexter, Bird and Ella in the '40's, Oscar in the '50's and Duke in the '60s. Mule and Slam even teamed up for a couple of records, including the ironically titled Shut Yo' Mouth in 1981.