https://segment.com/blog/customer-data-platform/
This is not surprising. The personalized SMS/push notification use cases (for which you need Segment data) are commonly implemented via marketing platforms (like Braze etc) as opposed to directly via Twillio.
I think this was the other way around. Twillio is powering a lot of these new age marketing automation tools. Instead of just being at the infrastructure layer, it wants to go up the stack and own the marketing applications. Sendgrid and now Segment acqusitions are part of the same strategy.
They did the same thing for the contact center (call center) software space. Lot of the cloud contact center companies were using Twillio and now they launched their own contact center product. That has been one of the key drivers in their 3x increase in valuation in the last 1 yr.
https://www.twilio.com/use-cases/contact-center
Segment's partners like Braze etc should be worried.