Kirby suds,
are mostly soap and air mixed with a little water. If done right, they make a carpet, couch and so on, slightly damp, then dry to a fine powder in about an hour. I did a carpet shampoo with every demo, and never soaked the carpet. However, a number of our trainees did! The toe touch control needs to be all the way down, if not, yes, you do soak the carpet.
The "do not use on wet surfaces" was a warning to protect Kirby from liability. Like the sticker on a pillow that used to read "Do not remove under penalty of law." That was finally changed to "do not remove, except by consumer." There are dozens of stupid warnings out there if you look for them.