Maybe they're having issues with his answer of "using an OS" which implies other people are not? IDK.
But as to you if you're running just one or two services from a machine you also use for other stuff using packages and systems services is perfectly fine. If you have dedicated hardware for it (or plan on having it), it starts to make sense to look at ways of making things easier for yourself in the long run. Docker solves lots of issues no one's talking about (because no one is facing them anymore), e.g.:
Different services requiring different versions of the same library/database/etc
Moving your service from one computer to another
Service requiring specific steps for updates (this is not entirely gone, but it's much better and it's prevents you from breaking your services by doing a random operation like updating your system)
Pinning versions of services until you decide to update without needing to sacrifice system updates for it (I know you can pin a version of a package, but if you don't upgrade it it will break when you upgrade it's dependencies)
Easily map ports or block access in a generic way, no need to discover how each service config file allows that, you can just do it at the container level. e.g. databases that can't be accessed from the network or even from within the host machine (I mean, they can obviously be accessed from the host system, just not in the traditional way, so a user who gains access to your machine on a user that's not allowed to use docker can't)
Isolation between services
Isolation from host machine
Reproducibility of services (i.e. one small docker compose file guarantees a reproducible host of services)
Endurance that no service is running as root (even if they only work as root)
Spin services in minutes to test stuff up and clean them out thoroughly in seconds.
There's probably many more reasons to use docker. Plus once you've learned it it's very easy for small self-hosted stuff so there's really no reason not to use it. Every time I see someone saying they don't use docker and don't understand why people use it I'm a bit baffled, it's like someone claiming he doesn't understand why people use knifes to cut bread when the two-handed axe he uses for chopping wood works (like, yes, it does work, but it's obviously not the best tool for the job)