Well if this was true "Every decent Linux distro has a package manager that covers 99% of the software you want to install" we wouldn't have to install it thru sh.
4/5 of the examples the author gave have a package in Fedora. The one that doesn't (oh-my-zsh) is simply a git clone so it doesn't make much sense to package it.
For one of the examples provided, rustup.sh, there were not complete packages the last time I looked for them. There were some packages on Debian and Fedora but I ran into problems configuring the Rust plugin for VS-Code because it assumed that the rustup executable was present and it was not (at that time.) Going down this path, one then becomes dependent on using rustup to update the Rust installation. Now I need to run two commands to keep my system up to date. (pip? Make that 4 commands pip, pip3. CPAN? Yet another.)
I have some confidence that at the least the Debian packages won't be changing rapidly, making it more likely that any problems will be discovered before they get to me. A script (or tarball) fetched and installed from the Internet can change literally from one second to the next. If a trusted site is compromised the next download could be tainted.
Not all "package managers" instill confidence. I've heard too many bad things about npm and the associated environment and won't have it on my systems, but I am not a web developer so the impact is the occasional utility I have to forgo.
Indeed. It must be the other 99% that aren’t in the repos because I have to install a lot of things that aren’t available in repos (or that haven’t been updated in stable in a long time).