This isn't true. The pid increments most of the time, but occasionally the new pid jumps down to a seemingly arbitrary value to start a new range. It is switching between unused ranges. This happens on my Linux machines all the time.

However, it doesn't matter for what the article describes: it doesn't depend on the pid always increasing, it only depends on the pid for each process being unique.

Pid reuse doesn't matter often, but it matters occasionally with how sigals are often used. Many scripts are written to signal a process that might have terminated already, e.g. using a pid stored in an environment variable or file. This usually just returns an error, but if there has been enough system activity meanwhile, the pid may belong to a new process, which the script does not intend to signal, so a random process may be killed unexpectedly.