Does SQL Server's is_nullable mean anything at all?
The answer to your question is yes.
That column specifies the default nullability of a data type, which can be important if the user doesn't express a preference e.g. in a CREATE TABLE statement with an explicit NULL or NOT NULL specification. Sadly, that information is only present in the documentation for the old compatibility view sys.systypes:

allownulls (bit)
Indicates the default nullability for this data type. This default value is overridden by if nullability is specified by using CREATE TABLE or ALTER TABLE.|

For example, one can override the default NOT NULL behaviour of the sysname type:
DECLARE @T table (s sysname NULL);
INSERT @T (s) VALUES (NULL);
SELECT s FROM @T AS T;





s




NULL




The timestamp type (preferred name rowversion) has additional special behaviours due to its intended use to detect row changes. See this other answer for some more details. You can still override its nullability in the same way, but it is not possible to actually store a NULL for a timestamp/rowversion attribute.
There are several complex layers to determining attribute nullability. Rather than repeat some of that detail here, I refer you to Why you Should Always Specify Whether a Column Accepts Nulls by Phil Factor for background information and additional reading.