AC560

I think you are asking the wrong question. As others have shown, yes you can descend down to 2100 based upon the clearance you have. Nothing I see written in the quote above shows the Captain questioning whether or not you could descend legally. It seems he just didn't want to descend yet due to concerns of CFIT. Thus the question should be even though we were cleared for immediate descent to 2100 why would the captain want to defer descent.

9780991975808

Points to ponder...

A clearance or instruction while on radar vectors goes beyond legality: it's also assurance the flight will clear any obstacles - if one has faith in the system, that is. If not, the clearance or instruction ought to be questioned when issued, not during compliance.

A decision is either adequate, or it's not. There can be only one perfect decision, but the number of adequate decisions is unlimited. The FO was presumably given control of the aircraft (PF) and he picked an adequate profile, IMO. It's like choosing to go left around a CB instead of right. Both are adequate - all else being equal. It might not be the Captain's choice, but if he interferes, he's micromanaging. If he thought the profile was unsafe, then he failed as PIC the moment he acknowledged (accepted) ATC's instructions.

When the Captain hit the Alt Hold, he in fact assumed control - it's his prerogative - but then failed to formally call it out. The FO should have simply replied, "you have control." Else, confusion is inescapable. Who is in fact flying the airplane?

A captain who uses his FO as an autopilot is ignorant of principles of delegation. And it's only a matter of time before his one-man show attitude will get him in trouble.

BTW, the way I read it, the OP's question was, "Does anyone know the answer and/or where I should look?" Don't see anything wrong with that.