/proc/pid/exe is also easily forged, without root. For example you can do LD_PRELOAD=evil.so /bin/foo on any dynamic executable, or spawn /bin/foo unmodified and inject code via ptrace or /proc/pid/mem.

I have a fileless, execless copyfail exploit that works by injecting shellcode directly into systemd's pid 1. (I should probably publish it at some point...)

Yeah the whole system is based on the ability of one task to apparently become another task, that's how Unix works. So the indicators in /proc are just that: indicative at best.

There's no reason the task should even be assumed to be executing code in a file. A process can map code into anonymous memory and continue executing there without even branching. Again this is considered a feature of the system rather than a flaw.