In the response under the “orderedLineRoutes” collection there 5 routes are listed at the 2 position the Line name is “Chesham to Watford” and its routes stations are listed as followed NaptanIds by sequence.
I think the junction north of Moor Park allows trains from the Chesham direction to get to Watford without going via Moor Park. I was surprised that there are any such journeys to be honest but they are very rare, hence no indication that there might be such a service on the map.
Whilst is IS possible for trains to use the “Watford East Junction”, they don’t for passenger services except for the very rare occasions when there is engineering work that does.
As a normal passenger, and I have done this, you have to go Moor Park and go from Platform 4 (Southound local) to Platform 3 (Northbound local) to get to Watford.
I’ve just had a look at the Datastore file for the Met. There is a journey at 05:16 from Chesham to Watford and the stop sequence for that is Rickmansworth followed by Croxley. No mention of Moor Park. The journey is advertised on Journey Planner so presumably in service rather than an out of service rolling stock movement.
So, while for 99% of the day (and 99.9% of all passengers) it would be necessary to change trains at Moor Park, there is at least one exception and the stop sequence would correctly exclude Moor Park and it must be the actual train movement that the OP is querying.
I can also see non-standard routeings specified from Amersham to Watford and from Croxley to Amersham, again probably for a single journey.
Services round the Watford North curve have varied over the years. You can find some previous dates on my web site at CULG - Metropolitan Line. During the recent timetables there was a period with a 2 tph service between Watford and Chesham that way, possibly by extending half the 4 tph Watford service.
Here’s your Bakerloo line as a Wikipedia track layout