The docs say:

In most cases this function returns the same string as lastQuery(). If a prepared query with placeholders is executed on a DBMS that does not support it, the preparation of this query is emulated. The placeholders in the original query are replaced with their bound values to form a new query. This function returns the modified query. It is mostly useful for debugging purposes.

Since it says that preparation is emulated on platforms that don't support it, I interpret it as implied that preparation is actually done on other platforms. Regardless the output of the method contains select ... where variable = :myValue ... etc.

This was mentioned in a comment on QTBUG-28883.