Re: How did they get 1000 merits?

I believe the merit score of any user can only be dated back to the last 120 days and after that it can no longer be traced.

In bpip, you can check the history of merits of a person, not only for 120 days.
Link- bpip.org
Credit- Vod

It's quite simple, some hero member got extra merit when the system was introduced, you can see in here theymos mentioning it:

Note that I later made an adjustment giving 500 extra merit to some hero members, but no extra sMerit was given here.

But why with only 552 activities? Any specific reason?

It might be possible that Rollin.io was a Legendary member but due to a huge number of deleted posts his activity went down as well.

BPIP also provides modlogs for each user. No post was deleted for that user.

This post explains why everyone got a different amount of sMerit. It's not the same.

He indicated this one.

Note that I later made an adjustment giving 500 extra merit to some hero members, but no extra sMerit was given here.