This was worked around ages ago in OpenBSD, and the workaround was already included in Debian (and by extension, Ubuntu) when I started maintaining it in 2010 (I no longer do).
Here's a link to a patch [1] from a version from when the package still kept standalone patches.
It's been a long long time and I honestly don't remember the details, but Debian's cron(8) still says [2]: Special considerations exist when the clock is changed by less than 3 hours, for example at the beginning and end of daylight savings time. If the time has moved forwards, those jobs which would have run in the time that was skipped will be run soon after the change. Conversely, if the time has moved backwards by less than 3 hours, those jobs that fall into the repeated time will not be re-run.
Edit: According to this bug report [3], this workaround first entered Debian in 1999.
[1]: https://sources.debian.org/src/cron/3.0pl1-137/debian/patche...
[2]: https://manpages.debian.org/unstable/cron/cron.8.en.html
So we can assume the author was not running a distro that had "fixed" the issue?
Most likely, yes. The author mentions vixie-cron, which was the name of the project before Paul Pixie joined/founded(?) ISC, and it was released as ISC from after.
Debian's fork is still based on vixie-cron, but it couldn't have been the one because of the aforementioned patch.