True, but Intel seems to finally be ready to start shipping 10nm CPUs in bulk by 2H 2019 that will work in the iMac and the next generation of Intel Xeon W for the iMac Pro will be announced in May at Computex and also should (hopefully) be available by 2H 2019, as well. So with them not really having a window to announce before WWDC in June, it makes sense for them to announce the iMac (iMacPro) will be using those CPUs and ship in 2H 2019.
No, AFAIK, the latest predictions have 10 nm shipping in late calendar Q4, for low power laptops.
ie. Even October 2019 is probably too optimistic for volume Intel 10 nm shipments, and even if it were to ship in October, it wouldn't be for the iMac.
So, most likely we'd get new 14 nm iMacs in mid-2019, and then 10 nm iMacs in 2020.
BTW, my original plan in 2014ish was to wait for 10 nm to be released in 2016 or 2017 to get a new iMac to replace my 2010. I'm glad I had to buy in 2017. It turns out the timing actually was pretty good. Had I not been forced to buy back then and instead decided to wait for 10 nm, I would have had to wait an entire DECADE.
Mind you, a 14 nm 6-core i5 would have made me happy... but it would have still meant waiting for 2019.