There's nothing surprising to the ranking being similar.
Old accounts don't use classic, they use the normal HN homepage like everybody else. Which means, they see the same top links as everybody else, and upvote them, like everybody else.
In other words, /classic is remarkably useless. It might have been useful when it was created (11 years ago!), if a sizeable part of those old accounts would use /classic, and only /classic.
I use /classic, slightly less dross. It’s gotten gradually less different through time, perhaps something to the point that others may not be using it.
I think it's supposed to be about the "startup" users. So you can't use a dynamic cutoff date.
Nowadays, HN is more of a general tech aggregator, but discussions where some users lament the turning away from exclusively startup content still happen every now and then.
(Personally I'm happy with how it went. Startup stuff bores me.)
So is there a reason to think now that the cutoff date is Feb 13, 2008, or could it be dynamically counting accounts over a year old?
0: https://news.ycombinator.com/item?id=607271