I’d argue against this point. Android integrates with Google products regardless of device maker and a whole lot of people use these data mining Google services all the time.
Google Assistant wanted to leverage that data to help you do stuff (including the holiday planning) but all it can reliably do is to automatically add stuff to your calendar and tell you about it in the morning.
Apple has a similar level of insight across users of its platform as long as those users do buy in across its services. If you use Google calendar instead of iCal, that's gone.
Daily stuff in a digital life amounts to death by a thousand cuts. When I plan a holiday I don't need an AI to go and book stuff. I want an AI that can compare stuff across some providers I use (eg booking.com, Airbnb, whatnot), choose something in line with my preferences (learned via interactions in social media, for example). I need that AI to act on my behalf and for example cancel a holiday when bad events happen and act on my behalf to obtain refunds or access insurance procedures as needed.
No providers will open up to Google. They might do so for an unaffiliated third party (at least until said party is bought or finds success, when I expect said providers to demand a slice of the pie).
Rabbit hints that they might do this. For me, it's too soon to tell but they have a better chance that big names.