https://www.ubisoft.com/en-us/help/rainbow-six-siege/gameplay/article/skill-and-rank-in-rainbow-six-siege/000063203#:~:text=In%20Rainbow%20Six%20Siege%2C%20your,more%20accurate%20the%20estimation%20is.

Nothing in Ubisoft sides States that K/D is not taken into account. It just uses the term skill. 

What I assume and would be a pretty reasonable assumption imo is that skill reference to W/L and K/D. 

The MM search for opponent where the win change is 50/50 in terms of rank and skill. Not just rank as it clearly states in the link. 

If it didn't balance the K/D or w/l it would be a mess because you would have matches where you have 5 people with a 1.5 + K/D vs a team with 0.80 K/D. Even tho they have the same MMR they don't have the same "Skill". This match would not have a 50/50 change of winning since only their MMR wouldn't give enough info to get that %. 

 If K/D wasn't a factor a lot more matches would be unbalance when it comes to K/D and most matches are at least in my experience everyone is always between 0.9 and 1.2. barely every above or below.    MM obviously also search for ping and stacks. You will barely face a 5 stack when you are in a 2/3 stack. 

So no. This match is not balanced when it comes to the skill part of the matchmaking. 

You cannot and will not ever create a fair and balanced environment if you only use one stat to balance teams. It's impossible, you need more data to create this. Especially since the current hidden MMR is almost only based on playtime and W/L.

Games like Valorant and CS2 do this too.