Host linux versions

I installed docker on Fedora 30 and Ubuntu 18.04. I understand the abstraction it offers, but was thinking how the host OS could influence things. For example, within containers running on each I see different kernel versions

Ubuntu:
root@dedd1b5bf7d0:/~# uname -r
4.15.0-58-generic

Fedora:
root@0091c8535089:~# uname -r
5.0.17-200.fc29.x86_64

What if there are bugs/optimisations, or just general differences between these. Are the apps really 100% insulated from them?