Very High Skill is 3.8k, so technically you're still in the trench.
I always understood the trench as players who don't understand the basic concepts of dota
earlier I was in a game where our support AA (after much begging from our carry) bought a sentry to deward the high stone pillar ward spot in the jungle that we pinged out)
he put a sentry up there and walked away...
Not bothering to cast his E on there to get vision to see if there's actually a ward there
so he doesn't understand how sentries work
that is an example of trench
another would be a carry in trilane who keeps autoattacking creeps breaking creep equilibrium and giving he offlaner tons of free exp and last hits under his tower (but then doesn't want to push the tower and doesn't understand how tower aggro works)
at 3.5 k mmr on eu I'd say about 90 percent of players at least know better than that...
I see a lot of de warding, a lot of creep skipping, scouting with illusions , stacking ancients and other things that I almost never used to see when I was newer at the game and playing with other equally shit players
Rikis do shitty in most of the games I see now because people buy sentries and dust from the moment he hits 6 (and have them for every tower push) and shit all over him
In the trench he'd be the pub stomper especially since the invis not breaking buff
When I think of trench I think of people who don't understand what they're doing or why they are doing what they do, and just kind of blunder through their games
When you see a gyro buy a helm of the dominator (which is in every guide) but actually use it to stack with then I consider that out of the trench