I got an answer from ICAO regarding this issue. As a reminder - I asked whether I can use 4 letter ICAO codes of airports in the Route section. The ICAO officer confirm that I can.
Dear Mr. Smid.
Thanks for your consultation.
Your interpretation on how to fill Item 15 (route) of the FPL form is correct.
Regarding the interpretation of “Significant point” I’d like to highlight that you also have the definition of “Reporting point” and in your case, the airport 4 letters designator is considered a reporting point instead of a significant point due to the definition of both.
Reporting point is a ”Specified geographical location in relation to which the position of an aircraft can be reported.”
Significant point , although has the same use, is defined as a ground-based navigation aid, an intersection and/or waypoint. And in this context, intersection is a significant point expressed as radials, bearings and/or distances from ground-based navigation aids.
As you can see there is a slight difference, in terms of definition, between the two, but again, your interpretation on the way to fill Item 15 is correct.
International Civil Aviation Organization
Air Traffic Management Section