Facel
This is the negative aspect of Debian: It’s package repository is known to be very stable, but since the major versions are frozen, those become outdated more and more. There are ways of course to add Buster (the next stable Debian release, expected to be released this summer, currently in testing repo) or backport repos, but actually I would no recommend if you have any other chance. Mixing repositories currently causes several issues on ARMv6 devices, where we were forced to add the Buster repo for PHP7.3 install.
Facel
Jep, apt upgrade includes libc6 updates to the most current that is available in your repos (Stretch then). dietpi-update includes the APT upgrade as well.