1. I think what he means is taking away agency by denying RAW. All the tools somebody like me needs to be a combat monster are in the PHB. The tools in 5E are far less than what they are in 3E or 4E, but there is more than enough to disrupt(to your standards, not mine) the table you seem to be implying. You are kind of implying that the DM has a responsibility to stop a player whose playing right out of the book.
2. How exactly does a player force a conflicting playstyle on a group? How does that player like that end up in the group in the first place? What about for non-homogenous or more casual groups?
I'm fairly sure Imaro was talking about encounter design. Nothing about changing RAW or player agency during encounters. There is a habit in forums to misinterpret an argument in an extreme fashion sometimes.
The DM's responsibility is to ensure everyone at the table is having fun. There are situations where the play of one player is preventing or disrupting the fun of the others. Its up to the DM to adjust things to ensure that everyone gets their fun. Flat out changing RAW or saying "You can't do that." in response to a
realistic action of one of other players however is not what was being talked about.
As I understand it, the situation Imaro was talking about is this:
Most of the group enjoy roleplaying and social encounters, one player very much prefers combat. She builds her character to be very good at combat, and because she enjoys it, will try to resolve as many encounters as possible through combat.
So: we have one player who is turning most of the nuanced social challenges into a sequence of to-hit and damage rolls. The other players do not find this fun, and would have preferred to resolve the encounter through non-combat means. However once Little-Miss-Combat-Monster has pulled steel and started killing, resolving the situation through non-combat means is unlikely. Therefore much of the game is no longer enjoyable to the remainder of the group. The only rules being broken is the implied social contract amongst a D&D group that everyone is here to have fun: no cheating or other rules being broken.
It is the DM's responsibility to change this situation so that everyone is having fun again. He can't (or shouldn't!) remove player agency by simply saying "You can't do that." when the disruptive player declares that they are drawing their weapon and attacking the merchant or guard captain or whatever. Neither should they just break rules by granting arbitrary bonuses to the captain's AC for example, in the hope that the rest of the party can bring the violent member under control before she lands a hit.
Instead. as Imaro seems to be saying, the DM needs to adjust encounters so there are less where starting a fight is a viable method of resolving them. There is almost always going to be some combat, where the butt-kicker can shine and get their . . . well, kicks. However this also allows the rest of the party the opportunity to enjoy themselves doing things that they like to as well.
Granted, the combat optimiser might prefer just a long series of combat encounters and find their fun a bit limited, but its probably the best compromise for everyone in the group.