It is the case the grandparent is referencing with IE bundling in Windows. It reapplies the antitrust laws to monopolies selling bits. If Google is found to have a monopoly in market X and uses that control to gain control of market Y by forcing out competitors without really competing, that is the type of monopoly that's being targeted. This is what the grandparent is talking about. It is different from a simple monopoly the parent refers to.

Having a monopoly isn't illegal. Abusing or attaining the status in certain ways are. Walled gardens can certainly be considered a form of monopoly, but that means nothing in the eyes of the law unless they use abuse their monopoly status.