You got some moderately bad advice.
Running programs will continue to use the libc version that was on disk when they started. They won't even know glibc was upgraded. If something is broken before rebooting, it'll stay broken after.
Running programs will continue to use the libc version that was on disk when they started. They won't even know glibc was upgraded. If something is broken before rebooting, it'll stay broken after.
For a trivial change to glibc, it won't cause issues. But there's a lot of shared libraries and lots of different kinds of changes in different kinds of libraries that can happen.
I still haven't nailed if it was due to a shared library update, but just the other day, after running upgrades I was unable to su or sudo / authenticate as a user until after rebooting.
Firefox on Linux did not really enjoy being updated while running, as far as I remember; Chrome was fine with it, but only since it does some extra work to bypass the problem via its "zygote process": https://chromium.googlesource.com/chromium/src/+/main/docs/l...