LegacyUser: man (or woman) of mistery...

LegacyUser is not an actual person, but rather is a special user account that was created to facilitate the transition from the old GetSatisfaction platform to the current Vanilla platform that powers FamilySearch Community. Because of differences in those platforms, posts and comments made in GetSatisfaction were generally attributed to LegacyUser as they were migrated to Vanilla.

Edit: This discussion is not visible to most people, so I removed the link; see another example in a later comment. As an example, consider this old discussion, which took place on the GetSatisfaction platform, but was migrated to Vanilla in 2020. You'll see dozens of comments attributed to LegacyUser that were clearly made by at least a dozen distinct people.

So the contributions attributed to LegacyUser do not represent a single incredibly active person, but rather a conglomeration of the former contributions of a large number of unnamed people. You may also see some post-transition contributions attributed to LegacyUser. This may happen because of administrative actions related to user accounts that are deleted.

As for the date, the Unix epoch is the timestamp used as the starting point for the digital storage of dates in many systems. It was 1 January 1970, which because of time zone issues is sometimes displayed as the day before: 31 December 1969. When a timestamp is stored as 0, it will generally be displayed as 1969 or 1970. That doesn't really mean the event occurred then, but rather that no date is known, so the value of 0 was stored.