I agree that this trail, with a length of 52,5km, could formally be seen as part of a regional network, if using the definition of the German wiki page. I did not find relevant definitions on the English tag definition page https://wiki.openstreetmap.org/wiki/Tag:network=rwn?uselang=en 
I think if you look at it from a network perspective, this is just one of a handful of all 1070 hiking relations operated by the SAT that go beyond 20km, but are not divided in daytours and are never exceeding 65km, and are all following the same systematics for numbering, so I dediced to see them all as part of the local network. I agree it could be discussed, this regards also the O300 (64km and was already lwn), O133 (44km and you have seen it yourself as lwn 6 months ago) and E349. So basically only the O500 was tagged with network rwn of all 1070 trails operated by SAT and even as it wasn't the longest.
Cheers
Martin