I saw a contribution by this maintainer to another user who ALSO HAS 14.5k followers: https://github.com/yumiaura/myCat/pull/99 "yumiaura" and a preference for "my[APPNAME]" repo naming.

What is this?

Are the histories that Github presents all derived from someone's uploaded git repo .. e.g. can I simply claim to have created a GIT repo in 1970, and the "github commit graph" will dutifully represent this claim in its green-colored activity graph?

yes, the github contributions heatmap is known to be open for manipulation. folks will use it to draw art by backdating commits.

if i have it right, only commits can be manipulated, other contributions should be safe — i don't know what is possible for orgs moving old discussion archives into github, see python's issues for reference

see: https://github.com/dspinellis/unix-history-repo

They're almost certainly not genuine accounts, maybe used for karma farming, phishing, social engineering, future malware distribution?