His point is what I've been saying for a long time on here.
In blunt terms, he's a central midfielder, without a central midfield being played as a second striker.
To get more involved as is, he needs to go AWOL drops wide because that's were Everton are instructed to play - Wolves+cross for Iwobi springs to mind.
Either than or start scoring bullet headers or hope for a half clearance lottery.
Not sure they do?
In terms of whether its Sigurdssons fault or the holding midfielders, I think it's Silva's. Both those holding players play most of the game in their own half and very rarely get up the pitch. Maybe that's why Silva wanted Doucoure as a box-to-box, we don't have one.
As for him not making himself available, I don't see how a player who literally covered more ground than any other over the course of a season can be told he doesn't make himself available... if he's not making himself available then could how we setup be a factor?!
Every attack we try an conjure goes down the wings and fans then bemoan a player like Sigurdsson floundering about on the edge of the box.
If you've played the game at any level, his position in the Everton team is the one you'd least be enviable to play.