Since its in the foyer, it may be better than flat, since that is where hands may touch walls etc, when coming in from outside. It looks fine in the pictures. I would probably leave it for now, and later if you just cant stand it, you could either repaint just that part yourself, or have the painters back. I would never put flat on the walls, I would have done at least eggshell finish, which is a bit more wipeable. I only ever use flat on ceilings only.

I wouldn’t have painted the shiplap in flat but would have specified either Matt or eggshell. I’m afraid this is on you though because the painters though technically not right in stating that the shiplap is trim it is however wood so one could assume it would be painted like the rest of the wood trim unless otherwise specified. I think you will find that once you have window treatments up and hang art the glare will be less noticeable. The paint sheen will dull slightly also as it is just freshly painted. They did do a very nice paint job . If you really feel it’s too shiny have them do that section in eggshell.