I responded "This is not true" to a sibling comment about this same topic, but about "shared libraries", which is the opposite problem (multiple programs could load the same shared library and try to interact).

This is absolutely not true for Linux kernel updating. While you won't be using the new kernel before rebooting, there's 0 risk in not rebooting, because there's exactly 1 version of the kernel running on the machine -- it's loaded into memory when your computer starts.

There's of course rare exceptions, like when a dynamically linked library you just installed depends on a minimum specific version of the Linux kernel you also just installed, but this is extremely rare in Linux land, as backwards compatibility of programs with older kernels is generally a given. "We do not break userspace"