A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
According to the Oracle of the Void, The Maws Grave is an unaligned stronghold, not Shadowlands. Though it states you cannot gain or lose honor, it technically can still win via honor since nothing about it explicitly states otherwise.
When playing cards like The Return of Fu Leng that look for clans that are exempt from winning via honor, this stronghold would not fall into that category and can opt out even though it is also Shadowlands.
Is this an error, or intentional?
According to the Oracle of the Void, The Maws Grave is an unaligned stronghold, not Shadowlands. Though it states you cannot gain or lose honor, it technically can still win via honor since nothing about it explicitly states otherwise.
When playing cards like The Return of Fu Leng that look for clans that are exempt from winning via honor, this stronghold would not fall into that category and can opt out even though it is also Shadowlands.
Is this an error, or intentional?
No error. The Maw's Grave (as well as The Forgotten Temple, one might note) is a delightful loophole, allowing you to renounce an ability to win by Honor that you DO have, even if your inability to GAIN Honor means you can never practically achieve it. Enjoy!
That said, I do feel obligated to correct your implicit and explicit assertions above: The Maw's Grave is not, nor has it ever been, a "Shadowlands" or "Shadowlands Horde" Stronghold.