The link you referred to said it for me

"A computed column can not directly access any column outside its table. This limitation may be overcome by using a User Defined Function"

The computed column is not referencing the column in another table, the UDF is doing that. I was another person who knew the answer but struggled due to the rigidness of the available options. Technically it is possible to get access to that data, but not without using an intermediate piece of code

Paul