Trenin was 3/4 line guy on SKA during the Covid stoppage, then stepped right into Nashville's lineup, The other guys were also 3/4 liners in KHL then stepped right into NHL lineps with no or very minimal AHL time the following year.
You said:
"if we want him to be in the NHL once his contract expires, he should be able to crack any top 6 in the KHL.".
This doesn't neccessairy need to be the case. Nothing more, nothing less.
He was already in Nashville's lineup before the stoppage. He was developed in North America. He had 6 years of NA experience; years where he was playing on a North American ice surface and developing specifically to be an NHLer.
It is pretty clear to see this is not apples-to-apples. In fact, St. Petersburg probably didn't give him top 6 minutes for that exact reason... They had top 6ers who were going to be there the entire year, and he is not exactly a special enough player to justify putting him in the top 6 over those guys if he is only going to be there temporarily. They are not going to mess with their lineup for a guy of his caliber.
Rather than listing a bunch of fringe or depth NHLers (many of which had to spend time in the AHL before cracking the NHL in any capacity), here are some good Russians: Panarin, Kaprizov, Kucherov, Kuznetsov, Kuzmenko, Mikheyev, Buchnevich, Tarasenko, Nichushkin, Dadonov... I could also throw in Barabanov in there too but he is not as good.
All of them, except Buchnevich once he moved to St. Petersburg, were top 6ers (if not the headliner) on their teams before coming to the NHL. And all of them played on upper-mid level teams (aka Magnitogorsk-level); not basement level. Buchnevich moved mid-season from a lower mid-tier team (in the top 6 of that team, a position he held for 2+ years) to a team with a top 6 that had 6 of the best KHL forwards in the league, so it was a bit tough to crack through that even though he was doing fairly well in a depth role there. Grebyonkin has 1 year in the top 6 of a basement team.
There is not much of a mid-tier when it comes to Russian forwards. Most of them are starters/top 4 defensemen/top 6 forwards or they are replacement-level and likely heading for Russia in the near future. Most mid-tier Russians choose to come to North America (Trenin, Barbashev, and Namestnikov to name a few) for better opportunities and can have nice, long careers. Some higher end ones, like Sergachev, Provorov, etc. come too. But at that point, they are developing like a normal North American prospect (CHL/USHL/NCAA then likely some time in the AHL and eventually the NHL if they are good enough): see Abramov and SDA. There is a reason that there are maybe a dozen draftable Russian prospects in each draft (this year is deeper for Russians and I still have less than 20 on my entire ~150 prospect board). That doesn't stop NHL teams from drafting double that number each year, and watching as almost all of them fail to even make it off the ground as prospects...
The ones that come straight from Russia are top-tier guys in that league before they come over, or they are going to the AHL and developing there (historically, most do not choose to do that, but it has become a little bit more popular among solid talents who are buried in deeper systems). Grebyonkin falls more in the latter category. He has shown he is at least worth developing in the AHL based on this year, which is more than most Russian prospects can say, but people really need to pump the breaks if they think he is anywhere close to the NHLer. Unless he takes a massive leap next year, he is still a few years away from NHL-caliber.