That doesn't follow at all. Yes the idea for the rule comes from the description of Talinie, but there is no reason whatsoever to say that the guilds in my kingdom couldn't follow suit.
VANILLA:
Optional devastation rule:
To simulate degradation of a province, the DM may assign it a "devastation point" during any domain turn in wich the guilds collect more than half (round down) the GB allowed by the province guild holdings.
For every 20 devastation points permanently reduce the procince's magic potential by 1 level.
If the guilds stop cutting and mining in a province the devastation will heal 1 point per turn. However, the citizens loss of income will cause loyalty in that province to drop by one grade.
So, if we run as close to vanilla as possible then the provinces will run out of magic. The guilds wont get anything extra by devestating a province, and the bonus for the landed regent is that he/she doesn't get a negative prosperity modifier.
So if you wanted to nurf yourself, guilds and mages in your land then you could advocate that the rule should apply to you also.
That we twist the rules to actually generate extra income for the guild is fine.
That we twist the rules to hurt the province ruler when the province is devastated instead of when it is not, that is fine.
That we look at how big a nurf bat we will hit wizards in the domain with, that is fine.
But not if your going to use the combined new possibility of a gain as an excuse to put it on every province in the play area with secondary terrain of forest or mountain.
If that is your goal I think we up should keep the rule that guilds only get half income from holdings in Talinie unless they devastate(or holdings count as half their level round down), and the province get -1 prosperity unless the guilds devastate.
In light of that I think the nurf to the wizard need to be as apt and prompt as -1 prosperity and half income to guilds.
Perhaps then -1DAC to all magic actions in the province when it has been devastated.
And leylines and hookups in a devastated province run at double upkeep.
But you could have a point that since we border the same forest the same nurf should apply to your land.