I wonder how accurate these statistics are. Total users are over 1mio now, but users that have been active at least once in the last half year are only ~45k. Which is not bad either, but a far cry from 1mio.
The 6-month-active count usually means "This user has been active at least once in the last 6 months" not "This user has been active for over 6 months".
So if you sign up today and spend today on Lemmy, then you have been active at least once in the last 6 months.
I know at least at the start some users were confused by the federation process and created 10+ accounts to "subscribe" to other instances plus there have been bots signing up through smaller instances without capcha or email verification so that might explain the discrepency