It's still not a message store property. It is, however, present as a row in the message store table, hence the restriction can work.
It's not always the case that a property present in the row of a table is also present in the object that the row in the table describes. This is one of those cases. You'll see several more cases like that in the attachment and recipient tables. The table rows can and do contain additional metadata about the object they reference.
It's not always the case that a property present in the row of a table is also present in the object that the row in the table describes. This is one of those cases. You'll see several more cases like that in the attachment and recipient tables. The table rows can and do contain additional metadata about the object they reference.