This was not true of YouTube, where Google actively blocked Microsoft from building a native client.
Google still disallowed it.
Google likewise disallowed various Amazon devices from accessing YouTube during their Amazon dispute.
Other entities like Instagram were under no obligation to build for Windows Phone, but 6tag was built and served the purpose. I don't believe Instagram blocked 6tag from working. If they did, it might not be anti-competitive, but just anti-consumer.
Beyond this, Google's behavior had ripple effects. Developers could tell that Google was putting up roadblocks to make Windows Phone an undesirable platform, and that discourages contributions.
If iTunes only exists on iPhones, but not Android, then it doesn't matter if it also doesn't exist on Windows Phone. Apple, the iPhone and iTunes are not monopolies. They are a walled garden, though. That's just a separate issue.
Is it me or does that just sound like whataboutism? So people complain about Google intentionally screwing over a competitor but what about Apple? Apple is very much "we play on our own court".
Windows phone had no Instagram too, and even that made it painful to use. If the platform doesn't have enough users, developers aren't obliged to develop for it.
You could always use a web browser for all of Google's services. They never blocked HTML5 apps.