I'm definitely very opposite of that -- adjustable difficulty, especially on the fly, is a genuinely excellent bit of design and one more games should be adapting more fully, imo. I'd frikkin' love if every game difficulty is a relevant setting for just slapped sliders down somewhere for the mechanical aspect of things and let folks go as wild or restrained as they please. Even multiplayer ones (though those on a map or round basis or whatever, probably). Have a default, balance that default around your envisioned design, and then let the players do whatever the hell they want to on their own time.
Confident, as it's being referred to here, design when it comes to mechanics is one of those things that can and does hard lock swaths of the population out of a game's content. Be it mechanical insufficiency (there's basically one proper bullet hell game I can beat without cheating, and that's Starward Rogue playing Redshift, which radically changes how the game plays in the exact way that compensates for my trouble with bullet hell design), time constraints, or whatever, when you design without some form of give you're, well... designing without give.
Some folks are basically going to be punished for daring to try your game, and that honestly kinda' sucks. Especially for folks that don't have the means or awareness of how to beat the shit out of a game that's being an asshole with a memory editor bat. More will be let down because there's just not a perfect design, some folks just really don't or do like something (grind, drop chances being low or high, damage/tankiness values, whatever) and it's going to be incompatible.
Rather than telling one set of folks to get stuffed, you can just... let them adjust it. If you're good enough at what you do, most people won't care to adjust the values, and for those that do it'll be a kindness (and if you're not good at getting the mechanic values right, well, giving folks the means to fix your screwups is also a kindness :V). That, to me, is confident design, a belief their stuff will hold up and be enjoyable regardless of what shenanigans the players get up to, as well as respect for their audience's time and preferences. It's good stuff, even if it maybe pulls back the proverbial veil a bit.