>> My guess is that nobody is going to notice this unless they are specifically looking for this technique.
But having two identical PIDs is a pretty weak cloak. Even more so when reducing terminal clutter e.g. run "ps | grep procname" ... anyone not completely asleep is bound to notice it.
Probably you read the article that was linked from this article, which does indeed make the process completely vanish, but leaves a suspicious empty /proc directory.
This article 'solves' the problem of an empty directory by simply bind-mounting another process instead - but that causes ps to output a duplicate line (including process ID) for the other process, in lieu of the process being hidden.