Treble devices not same as A/B devices and vice-versa. Why? This XDA blog How A/B Partitions and Seamless Updates Affect Custom Development on XDA has some Q&amp;A's, of which :


My device supports A/B partitions, does that mean I can make use of a Project Treble Generic System Image?

Not necessarily. Project Treble and A/B support are unrelated. The Motorola Moto Z2 Force doesn’t support Project Treble, yet it supports the A/B partition scheme.

My device supports Project Treble, does that mean I have an A/B partition scheme?

This is not always the case. The Honor 9 Lite is a prime example as it supports Project Treble yet does not have an A/B partition scheme.

(Emphasis added)
I thought Treble and A/B devices are synonymous in the sense having one implied other. That's not the case
Why is this? What are the differences?
Both technologies have nothing in common, really, so everything is unrelated:

The A/B partition technology is the horizontal separation in the following image, whereas Project Treble defines the vertical separations inside each partition.


Project Treble diagram source

A/B partition scheme

Basically the A/B partition scheme for "(Seamless) System Updates" is nothing more that two operating systems installed side-by-side, each in its own system partition. The unusual part (compared to e.g. having multiple OSes installed on a PC) is that both OS installations use the same user data partition. Also, when installing updates the active OS installation applies modifications to the other, inactive OS installation, instead of to itself.

The A/B partition layout does not make any assumptions about the OS that is installed inside the partition, hence it does not matter if the OS is Project Treble compliant or not.

Project Treble

Project Treble applies to the internals of each OS installation and how the different layers within the OS communicate with each other. Hence for Project Treble it is totally irrelevant if there are one, two or many OS partitions. The A/B partitioning scheme is compatible with, but separate from, Project Treble, and vice versa.