I certainly wouldn't say that. Full Framework .Net and .Net Core are _far_ more similar than they are different - they share a very large common base class library API surface in Net Standard.

A huge percentage of .Net MVC 3-5 (full framework) applications could be migrated to .Net Core MVC in a matter of hours to days.

You don't even have to specifically target .Net Core in a lot of cases - v2 included a compatibility shim so that libraries targeting full framework just work as long as they don't call any of the API's that were in full framework but are not in the Core runtime.

The main differences are in startup configuration of your web app and how you go about implementing cross-cutting concerns that might inspect or intercept every request - but even for a lot of common things like authentication there are similar extension points to what existed previously.

There are some particular technologies - like WCF Server or Linq2Sql - that are hard blockers if you were relying on them - but by and large moving from older full framework .Net to .Net Core is not that difficult and certainly doesn't require learning much new, especially fundamentals - it's mostly just some details of the web app framework.