I think the word "humming" is misleading because it's an activity usually done with a closed mouth and with the breath coming out of the nose - also not advisable while playing.
You are correct of course, however you could argue that the mouth is "closed" by the saxophone mouthpiece.
However what matters is that you use either the throat action you use for humming (except with the saxophone in your mouth) or singing a vowel. It doesn't matter which vowel, as no singing actually comes out of your mouthy because there is a mouthpiece in there.
It's important with growling to not think you need to actually growl. The humming or singing will do.
But one difficulty id that when people say humming they think of a very gentle kind of hum, but growling on the saxophone is usually quite aggressive, so think of a loud hum, but without it actually turning into a Marge Simpson growl in the throat.
That along with the saxophone playing should do the trick of setting up the acoustic interference between the two note which cause the effect.
In my exercise I specifically don't specify a specific pitch (note how specific I am about that), because I believe it's best to do it by trial and error as each note reacts differently and each person is different.
Of course it's also important not to try and sing the same note as the one you are playing.