It's not "bro science", unless you count nearly every government health agency as part of "bro science". More lean muscle mass => higher basal metabolic rate. The bro science part is usually overstating the effectiveness of this, but it's a real effect.

Well, as long as the only source I can give are r/fitness and HN threads I can only show bro science level evidence.

Exactly. How come it's not given more importance? Is the effect not big enough? Seems to me it's the one that would dominate everything else. You're not-exercising during a bigger part of the day than exercising, after all.