Firstly, between those 'big four' browsers share > 95% of the market. So yeah - big four. Also it doesn't make sense to compare Edge to browsers that don't run on the same platform, so wtf would you include Safari - it doesn't make sense. If you factor out Safari, the remaining browsers make 2% of the market. So that description was entirely appropriate.
Secondly. it doesn't matter what the reason is. Does it matter whether its a technical limitation? It isn't Microsoft that is stopping its competitors - it is them stopping themselves. Microsoft hasn't magically disabled 1080p in other browsers - they did it themselves.
So Microsoft is right, and that's all people will care about if they want 1080p.
Also, Chrome sucks at memory management.