Comment by NekkoDroid

> systemd parses your crontab and runs the jobs inside on its own terms

systemd itself only has 2 references to "crontab" in its entire codebase and both of those are in man-pages.

My educated guess is that some other package is installing a generator to generate systemd units out of the crontab (e.g. https://github.com/systemd-cron/systemd-cron)