Holy strawman. Where are these people calling him a top 6 lock? I don't recall anyone saying that. My recollection is that a small number of people pegged him with top 6 upside that they thought he had a good chance to reach, but I don't recall anyone saying it was a lock. I haven't seen anyone calling him a top 6 lock this summer. Meanwhile, you are making definitive statements that his 'best case upside' is a guy who didn't get meaningful NHL time until his D+5 season, cleared waivers at the start of his D+6 season and then eventually landed a 4th line role after spending some time on a new organization's AHL team.
Surely you understand that there is a gigantic gap of potential outcomes for Neighbours between "best-case upside of Klim Kostin" and "top 6 lock." There is almost an entire NHL-roster's worth of roles/outcomes between those two things. Klim Kostin has 32 points in 103 career NHL games with a career-high of 21 points (in 57 games) that was achieved on the back of a 19.6% shooting percentage. His career ATOI is under 10 minutes a night and he averaged just 10:04 in Edmonton. They played him 9:33 a night in the 28 games following his 22 game heater that saw him shoot 39% and pump home 9 goals. They didn't qualify him (reportedly worried about what his 21 point season could net him in arbitration).
No one is calling him a top 6 lock while you are saying that his best-case scenario is that of an NHL/AHL tweener or 4th liner. By far the most prevalent opinion on him around here is that he could develop into a middle 6 guy. If you go back and look through the 'rank our prospects' threads you will see that the Neighbours supporters were basically all some variation of 'high floor, low ceiling with a best-case scenario being either 3rd or 2nd line.' It's 100% on you if you interpret that as people calling him a top 6 lock.
The 4 players you mentioned accumulated a combined 6 NHL games by the conclusion of their D+3 seasons. Neighbours got a 9 game tryout in his D+2 season and played 40 NHL games in his D+3 season. When he was sent to the AHL for 23 games, he scored at a higher point pace than any of the 3 guys you listed who played in the AHL during their D+3 season. Which of them had a great D+3 season?