There is no mention on this page that if you send a table containing non-string types and instances (i.e. parts) as keys, they will be converted to a string format. This should be added under the Parameter Limitations section as a subsection about table key type limitations.
No, that refers situations where there is a table with elements in it indexed by mixed types (both numbers and strings). The issue in this thread is that there is no mention of if you have a table indexed by an instance that the key will be converted to a string instead of being preserved as an instance.