While I understand this and also thought about this to be the solution, I still think that the way they formulated it is rather unfortunate.
However, the support replied to my request and states that it is truly intended:
"Hello,
Thank you for contacting the Guild Wars 2 Support team.
About your inquiry, it's intended that some items are currently not on sale. Therefore, if the item does not have a discount, this means that it's not included in the sale. We hope this clarifies your query.
If there's anything else we can assist you with, feel free to let us know."
So, we have to pay full prize (and wait for the rotation in the first place π)