@MMOtte - Yes, I am aware of this unfortunate fact! I am the one that made that edit after a discovery in collaboration with the Modding of Issac discord server. Because of the way the game calculates your planetarium chance internally (not visible, nor modifiable by modders, however discovered through datamining), you will not gain any additional % for skipping the treasure room in the mirror world, however you will lose % for entering it.

The wiki is a little misleading in that it says your chance "increases by +20% when a Treasure Room is skipped", however it is actually calculated in such a way that you gain +20% for every floor you enter (40% for XL floors) and you lose 20% for every item room entered.

If we look at the formula provided in the references section of the wiki[bindingofisaacrebirth.fandom.com]:
(0.2 * (stage - treasurerooms_visited - 1)). The c and d stage variants, such as Mausoleum and Gehenna, are treated as stage + 1.[i/]
and place values for what would occur if we entered the mirrored Treasure Room (avoiding all others), (bearing in mind that Dross/Downpour is a c/d stage variant, and that the stage value for Dross/Downpour is equal to basement[wofsauge.github.io]+1) when calculating the chance for when we have just entered Downpour2/Dross2 (as the chance is calculated on level generation, as soon as you enter the floor), we get:
(0.2*(3-0-1)) = 0.4 (+1% base value) = 41%
Say we then enter the Mirror World and collect the knife piece, then head to the next floor (Caves1)
(0.2*(3-1-1)) = 0.2 (+1% base value) = 21%!

To make this a little easier to understand lets simulate a run
formula: (0.2 * (stage - treasurerooms_visited - 1))

Entering Basement1
Chance in Basement1 = (0.2*(1-0-1)) = 0.0 (+1% base value) = 1%
Ignorning Treasure Rooms and heading to boss room

Entering Downpour1/Dross1 (Alt Path)
Chance in Downpour1/Dross1 = (0.2*((1+1)-0-1)) = 0.2 (+1% base value) = 21%
Ignorning Treasure Rooms and heading to boss room

Entering Downpour2/Dross2
Chance in Downpour2/Dross2 = (0.2*((2+1)-0-1)) = 0.4 (+1% base value) = 41%
Grabing both the regular Treasure Room and the mirror Treasure Room

Entering Caves1
Chance in Caves1 = (0.2*(3-2-1)) = 0.0 (+1% base value) = 1%

Even though we skipped two item rooms! The main issue here is that Dross2/Downpour2 both has the same stage value as caves1 in the formula (whilst technically being an alternative to basement2) and that the game treats the mirrored Treasure Rooms as completely seperated Treasure Rooms from their original counterparts (you can open the console and use lua print(Game():GetTreasureRoomVisitCount()) to see how many the game itself is calculating with.

AS WELL AS technically not increasing the stage value when you complete Dross2/Downpour2 as The c and d stage variants, such as Mausoleum and Gehenna, are treated as stage + 1 so 2+1 (Downpour2/Dross2) is the same as 3 (Caves1).