You obviously CAN guarantee ordering, it's just that you can't guarantee it as the sender, you need cooperation from the receiver. Additionally, putting them in a receive queue on the receiver doesn't solve the issue unless the receiver takes extra care to also read from the queue in strict (non-overlapping) order which is rarely the case, and even then has significant throughput implications. So it really is all on the receiver. This piece was written from the context of the sender.
Timestamps definitely don't solve the issue, I explicitly said to use a centralized sequence number if you must (not a great idea in most cases). Thin payloads: the idea behind that is essentially to use the webhooks as a "please update" kind of notification and then you get the most recent data from the server. Essentially what you called a "pull system", it's a combination of both a push (webhook) to know when to pull, and the pull to get the data. This also doesn't work as nicely in many scenarios (because oftentimes, receivers want the data immediately without having to fetch), but it's good in others.
Please take a look at the content of the article (rather than just the title), I've addressed most of it there too.