The aim is 100% compatibility, within the constraints of using what has come before in Open Game Content.
The classes I've designed work differently to 4e classes (they can always use use primary and secondary abilities, so MAD is never a problem, and they get access to lists of powers instead of having class-based lists) but these are built into the classes, so a 4e class can be dropped in with no changes. Similarly there's some sample extended challenges that break the skill challenge format, but a 4e skill challenge would still fit.
But you're right that the actual game material from 4e - the fighter class, the come and get it power, the needle fang drake swarm, etc - isn't reproduced. I don't have any plans to do so, both because of the work involved and the copyright infringement risk.
But (1) that shouldn't affect most of the third party supplements that might use Orcus as a base and (2) I think this is less important for a d20 System game than an OSR one. An OSR game not having the fighter is, I think, closer to a 4e-inspired game missing a martial defender. After all, an Essentials only game would be missing "the fighter" too (though it would have two reimaginings of it).
That said, I'm certainly not opposed to someone else reproducing this stuff!