Your initial interpretation is so far off the mark it's not funny. In no way, shape, or form did I mean that.

Your second one is closer, but all I was saying is that fieldset means a group of input, select, or textarea that have commonality such as being able to be interacted with and/or change the values of.

Form controls (buttons) having static values (if any) have no reason to be grouped that way for the user. Same for hidden input. Fieldset is an accessibility aid for user input. SUBMIT and RESET don't input anything, so I don't put them there.

These days I like to put all my SUBMIT/RESET, and hidden INPUT into a FOOTER. It makes sense, means less classes... and my braille reader seems to like it.

it has nothing to do with changing the behavior of input, select, textarea, or button. It has to do with aiding non-visual users with cues such as "landings".

Basic semantic markup.