No they are not single CPU Xeon E3s top out at 4-core designs. You have to move up to E5 to get 8-core.
The i9 is a consumer enthusiast processor. The Xeon equivalents and their pricing has not been announced. So we don't know anything yet about price points or whether it is an E3 or E5 part.
If these are the i9 equivalent Xeons the price points will be higher because i9s can't run ECC RAM. We also give no idea what chipset/socket this would use.
Most likely these are the rumoured Xeon E5 v5 chips (purley) which are rumored to have a TDP of 135 Watts, and be roughly based on the same Skylake platform as the i9 but support a different socket/chipset and add ECC Memory.
I would expect the 8-core Xeon used in the iMac will reatail for ~$1200-$1500, and be less crippled than the i7/i9 chips with regards to PCIe lanes. The iMac Pro will need 40 PCIe lanes at a minimum based upon the already released IO.
Don't confuse what a chip is capable of supporting with Memory and what a vendor Apple in this case is willing to support in standard product configurations.