I think he has saying the "pro" 2018 on the context, even trough he said just ipad 2018, anyway, the iPad Pro 2017 gets a cpu benchmark (geekbench) of about 9k, the iPad Pro 2018 gets 17k, CPU wise its almost the double, we need to see the new A13 at september to see how much of a improvement we will see on it. besides, even as it is now that's CPU power for years to come.
but.... what apple is using as the bottleneck for updates is the ammount of RAM, so that means if things continue this way we will see the end of the support of both together as all pros have 4gb of ram, and seeing apple released new devices this year with 3gb of ram, I cay we will see support for both for a long time still. at least 4 years, now we need to see how the pro 1°gen will hold on the 2021 update, as it still has 4gb of ram, but CPU power wise will be way too low compared to newer devices, if CPU proves to be a burden for it we will see it last update on 2020-21, if it continue to be supported 2022+, we can say that both the 2017 an 2018 pros will be able to hold updates for a looong time together (as it will be RAM based).