The meaning of “dark sede” is, in their theology, everything… all trad Sacraments are invalid.
Can you provide a quote?
It is my understanding that they no longer assist at any mass, but I do not believe it is because they think the trad sacraments are "invalid". At best, if they are truly "home-aloners", then they would think the sacraments are "illicit" and that it would be a mortal sin to assist at one.
I used the search function on their site. When you type in "Invalid", you get articles pertaining to the New Rites. When I typed in "sacraments", I found this file. Unfortunately, it is dated 2014:
Where To Receive Sacraments (schismatic-home-aloner.com)
Here, their issue focuses on what the priests teach, not their ordination rite.