People tend to over-summarise patents; while the subject of the patent is immutable types, the patent is on a specific method for implementing immutable types. It works a little differently to that method, as it's a type-inference and permissions-checking system.

As with every trivial patent, the valid objection is not that this has already been done, because every trivial patent is somehow a bit different to things which have been done before. This particular formulation allows structures to be changed while they are being constructed, but makes them immutable once construction is complete.

The objection to this patent is that the differences to prior art are all trivial and obvious to somebody skilled in the art: if you tried to make an immutable type for C# then you would get something that looked pretty much like this patent. On that basis, the patent should not have been granted.