It is very interesting that by adding the T2 chip they did a complete (?) redesign of the internals. If this is the last intel iMac, why bother with so much effort and resources? Is there any chance that we are going to have more intel iMacs in the future? even rarely?
With the iMac Pro already on T2 how much effort is required here? Apple also already has "in the bag completed" Mac Mini T2 , and Mac Pro T2. The rest of the entire desktop line up was already on T2. Where is the "Moonshot, rocket science, Buck Rogers" project here ? ( the Pro 5300 , 5500 , 5700 have been used in other Macs at different clocks to so no "bleeding edge" GPU driver work here either. ) Yes, there is some work to do here (different socket , different PCH, hodge-podge T2 NAND layout ). However, it really shouldn't be that complicated though that they hadn't done previously on other systems. .
The board layout is a bit different. There are fewer TB controllers to deal with. They can pilfer the 1 vs 10 Gb/E solution from the Mac Mini. The camera is off the iMac Pro; a camera from practically the same case. The updated SD Card reader was going to be show stopper problem?
This design should have be almost done last June-July ( in 2019 ). Pretty good chance Apple has just sat on it for more than several months. Even more so if they know they won't have a large scale iMac class Apple Silicon SoC to go to in most of 2021. (i.e,, the 21-24" iMac will use a different SoC than the 27" ). In that context, they would have even bigger motivation to 'slow roll' this as they have to fill in a gap.
If Intel and/or AMD delivered the binned parts a bit later and there were hiccups due to the pandemic ... the slide to this point isn't very surprising at all.