When playing "lap-style," a technique often employed by dobro, pedal-steel and lap-steel players, the slide is usually held by its outer surface, using the thumb and one or more other fingers, and pressed down against a fretboard that is facing up.

The "bottleneck" technique used by blues players who hold the guitar in the more conventional manner, with the fretboard and soundhole oriented vertically and facing out and away from the player, involves inserting ONE finger into a tubular device (made either of glass, like a broken-off-and-sanded bottleneck or a factory-made slide, or of metal, like a short piece of pipe).

So, yeah, old Hound Dog ~ like very many blues guitarists, old and young, black and white ~ absolutely holds/held his slide with a single left-hand finger. (Not sure whether to use past or present tense: is he still alive and playing?)

Incidentally, many players who "wear" the slide this way on a single finger (ring or pinkie) use the remaining two or three left-hand fingers to play some notes/chords in-between other notes/phrases that are "slid." Much of the Robert Johnson songbook consists of numbers that require this technique. So, a sixth finger on the left hand could indeed be an advantage for a slide-guitar player. Hound Dog Taylor, however, used the slide pretty much exclusively, rarely if ever using a bare finger to fret a note. That's how I remember his playing from when I last saw him in person ~ but that was back in 1968!