In light of this rank-70 guild, I believe the mechanism you're thinking of is for a very different beast.
What I think we're seeing here (a glitch)
When GW Registration first opens, the first few guilds to register get temporarily ranked 1st or 2nd etc. For example, a guild which is actually in the 70's but registers very quickly when only 6 other guilds on the whole server have reg'd yet, is Ranked #7 at that moment in time. Other guilds will supercede it tho, some also being labeled #7 or whatever. The mechanism that is live during open registration does not reliably self-correct in real time. Instead, after Registration closes, but before the first ticket is released, a server side script resorts the complete/closed list. Somehow that mechanism missed one. FWIW I'm trying to encourage an investigation of the root cause. But the decision/results thereof are out of my means.
The usual 13 Defs (intended behavior)
If a registered guild disbands or becomes un-reg'd for some reason, that guild still occupies a slot in the matchmaker matrix. But yet they aren't allowed to participate. When some other guild would normally be attacking this disbanded guild, the system substitutes a proxy. That proxy will see this battle in their reports, but it will not impact their crowns, win or lose, and regardless of whether or not their 12 regularly scheduled defs happen. The attacking guild will gain/lose crowns as normal based on their performance against the proxy.
I'm not aware of any way that a guild would get 13 attacks.
ETA: both above conjecture are based on personal observation of the game.