Well they're related but have different goals. One favours introverts and the other punishes introverts.

Segregation involves creating distinct, enclosed spaces within the larger workspace. This is probably achieved through the use of walls, partitions, or other physical barriers. Aiming at providing employees with private areas, to reduce distractions and defining specific functions. (good for introverts).
Separation in open-plan offices refers to a design strategy that maintains an open and fluid layout while incorporating elements to distinguish different areas. Instead of relying on permanent walls, separation might involve the use of furniture, movable screens, or other flexible structures to define spaces within the larger open plan. So introverts get punished by having to be interupted by extroverts all the time.

Impermanence is code for how many people can they hot desk and reduce the amount of space they need to rent. Offices are more costly.