In the video, you can see even more of those sketches. The variety in the video does not even show all of the sketches that our artist did. She tried out a variety of styles and levels of dress. It's a high-level, low-detail pass on character ideas.
Does that mean there will be muskets in the game, or is it all just a terrible, terrible mistake?
How are muskets a mistake?
Their inclusion would mean this is not a D&D game
(the 2nd Ed Php did have arquebus weapons but no one used them)
It isn't in the first place.