From your reference, I think this supports the other meaning:

The verb "monopolise" refers to the process by which a company gains the ability to raise prices or exclude competitors.

As I see it, without that ability to exclude, it's not a monopoly.

In a market without lock-in, or lock-out depending on your PoV, I can start selling widgets at any time and will do so whenever your profit margin gets too sweet. You're competing against me even if I'm not actively selling yet.