Why does Apple recommend a 2TB FD over a 1TB Fusion Drive for iMacs with 32GB of memory?
The second part of that statement "or all flash storage" is universally true. Solid state storage that ships with the current (2015) lineup of hardware is almost an order of magnitude faster than HDD technology. If you can make a justification that the increased performance is worth the uncharge then a pure SSD storage chain is a first choice/no brainer solution.

As for the 1 TB vs 2 TB fusion drive - there could be two items in play:


If the SSD component of the Fusion Drive is manufactured as faster than the 1 TB version, that would be a clear win. The page lists the 1 TB flash component as 24 GB of fast flash. The 2 TB drive contains 128GB of fast flash which is 5 times the space for fast file storage.
Just common sense. Internal iMac storage is not user serviceable so if you know you are a power user and need 32 GB of RAM, you likely won't mind the incremental cost of SSD or larger fusion drive and would statistically be someone that would want more storage in a year according to Apple's marketing research and experience.


I wouldn't think the sleep image usage would matter on 10.10 or 10.11 for several reasons:


The image is not purely based on the size of total RAM. The OS thins the sleep image through memory compression and other virtual memory enhancements.
Writing a sleep image file is a large sequential write, which is the one benchmark where a traditional HDD performs quite well as compared to SSD since the drive cache and firmware to write a large chunk of contiguous storage to a spinning drive is quite optimized. SSD really shines for random small IO where the HDD seek times and latency causes large slowdowns.