What do I make of unlinked tables in the same database?
No, there is no requirement for tables to be related to one-another.  The fact that you do have two tables separate from the other thirteen would be a code smell and you should be sure that you have not forced two different business objectives into a single database.  If the two really do support genuine use cases for the others then there's no problem.

A typical example are "staging" tables.  Here data from an external source is loaded into a stand-alone table.  Then that data is validated.  Only then is it copied across to inter-related tables.  Another is to hold system configuration or user options.  These have no foreign keys to the "business" data but support the operation of the system which uses that business data.

Lastly I would point out that the logical and physical designs can be different.  The data model (logical) may document foreign keys but the data base (physical) does not declare foreign key constraints for one reason or another.  This would be a very specific choice for particular circumstances and relatively rare, however.