It's on screen and has not yet been contradicted on screen, it's canon. If Picard saying Starfleet isn't military is enough for people to declare Starfleet non-military, then a computer graphic saying Stamets is Discovery's chief engineer is engineer is enough to declare he is the chief engineer.
In what way is Picard not TNG's lead, or Sisko not DS9's or Janeway not Voyager's? The shows may have an ensemble cast but the captains were very much the centre of their shows. TNG might have started with the intent of Picard and Riker being "co-leads" but Patrick Stewart eventually established dominance and became the sole leading man of the show. Sisko is very much the main character of DS9 and to suggest otherwise shows a very serious misunderstanding of the show and or the character. The series is built around him, he's the commander of the station, religious icon to the Bajorans and later war hero and key strategist responsible for defeating the Dominion. Voyager, I grant you, probably got closer to TNG's "co-lead" thing between Janeway and Seven of Nine, with the Doctor arguably coming close to the second tier, but I do not see sufficient evidence to declare Janeway isn't the lead.
Out of curiosity who do you feel are the leads of these shows if not their captains?