Sadly, as of 26.01.2023, the developers have decided to reduce the experience gained from defeating enemies from within the Cobalt Assembly. The method to power-level your main or alt characters via the Cobalt Assembly is now worthless.
The news was revealed this morning via the hotfixes on MMO-Champion. [Source]
Some may be aware by now, but for those unfamiliar, you can gain fantastic experience from power-leveling your alts from level 68 to 70 via the Cobalt Assembly. I must emphasize the term alts; if this is your first character to level 70, concentrate on completing the campaign first!
Your first character to level 70 should complete the campaign; this will give you the freedom to level via alternative methods (like the one in today’s blog) and make World Quests available for your alts!
[EDIT] As of 27.01.2023, the experience gained from killing mobs within the Cobalt Assembly has been significantly reduced (nerfed). This is no longer a viable option to power lever main and alt characters. [SOURCE]