Treble devices not same as A/B devices and vice-versa. Why?
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.