To the OP (and other posters that managed to connect successfully at 5Ghz): the fact your 1520 manages to connect to a 5GHz SSID doesn't mean it is necessarily connected at 802.11ac speeds. The best way to confirm is either successfully connecting to a 5GHz/80MHz only SSID (if your router allows the configuration of one) or use your router/controller monitoring section to identify the connection.
I configured my router for just 5GHz/80MHz and it "worked." I didn't do anything other than an internet speed test from a room on the far side of a small house. I also forced N+AC only instead of auto configure.
Interestingly
my phone could NOT see the 5GHz/80MHZ channel in airplane mode! I had to have cellular on! I turned cellular on and disabled cellular data and it could still see the 5GHz/80MHz channel. Initially I put the phone in airplane mode and just turned on wifi to do my wifi testing. But for whatever reason the 5 GHz/80MHz setup doesn't like that with my router/phone combination.
What I was talking about is the fact that Hidden SSIDs do nothing to prevent connections and do not enhance security in any way.
Does it really matter? The reality is hidden SSIDs exist on routers we need to use and do not control. As a side issue the academic discussion is interesting but if I need to connect my Lumia 1520 to a hidden 802.11ac 5GHz network whatever the "right" answer is is going to be of little comfort if I can't connect.
What irks me is when my girlfriend had a problem connecting to WPA2 networks with multiple iphones using multiple OSes the "geniuses" at the Apple Store just kept saying well it connects to our open network so the phone is fine. Online people suggested using a less secure encryption protocol. All those solutions were unacceptable. For one thing they exposed her to more security risks and even more pertinent was she wouldn't be able to connect to numerous networks... such as mine. I wasn't about to downgrade the security on my router in a crowded urban environment just to accommodate an iphone.
Even if we accept the premise that openly broadcasting your SSID 24/7 doesn't open you up to any more security risks it still doesn't address what you do if you go to a coffee shop, bar, friend's home, work, etc. that has a hidden 802.11ac 5GHz network. I don't like working with misinformation nor having a false sense of security so I appreciate the contributions challenging the effectiveness of hidden networks from a security point of view but there are other practical real world considerations I have to take into account before abandoning my efforts to get it to work.
Anyway I will keep tinkering and report back.