By definition of the UInteger, Integer and Number they are always encoded as a Variant.
see https://reference.opcfoundation.org/Core/Part6/v105/docs/5.1.6

The question is are they considered abstract types as below?

The text for Part6 / F.13 Allowsubtypes states:

This field shall be TRUE for all abstract DataTypes where the possible subtypes have different encodings.
When TRUE, the field’s value is allowed to contain subtypes of the DataType. All subtypes of Structure are encoded as an ExtensionObject (see 5.1.6). All DataTypes that are not subtypes of Structure are encoded as a Variant (see 5.1.7).

The sample code for this complex type in the UA .NET Standard reference server is compiled with Modelcompiler.

If the AllowSubTypes must now be set, then there is a backward compatibility issue because older servers were already using these types encoded in a DataTypeDefinition with AllowSubTypes set to false.

I think AllowSubTypes should be ignored for these datatypes.