They still need to interop the old async with the new async, without requiring recompilation of everything, otherwise it is going to be another .NET Framework to .NET Core, which is still ongoing.
This means that all existing libraries will continue working as is, and newly written or existing code that is compiled against e.g. C# 14/.NET 10(?) can automatically opt into new mechanism without user intervention. The write-up does cover the considerations for interoperability between these two models as well.
In fact, this will make using async from other CIL-based languages much easier as the burden of capturing the state and suspending/resuming the execution will be on the runtime rather than language compiler authors.
Something that I hope they take into account, having new code calling into the old code, is the easy part of the problem.