It's easily one of the most popular series in comic Cune right now. To the point where they had Tamamusi do the cover for their Yuri Drill anthology, which featured a large number of their artists, and even released it on the same day as the most recent volume of Amnesia. As well as advertising them together pretty heavily. They clearly expected Amnesia's popularity would help it sell.
I don't know what other Cune series could be selling so much better as to just call Amnesia's sales "okay".
You're effectively arguing this like it's supposed to be a given that there's multiple series in Cune that are selling "absurdly" well. Cune hasn't had any genuinely big sellers yet.
Yuri Drill originated as Tamamusi's idea, so of course they let her do the cover and it got promoted with her series.
What Tamamusi said of Cheerful Amnesia sales after vol 3's release:
https://twitter.com/tama_64/status/982750413735706624
明るい記憶喪失の方も好調だそうで一安心です^^
So good enough for peace of mind for now.
No, I'm arguing that compared to other series in Cune, it is selling extremely well, and because of that, it isn't in any danger of ending.
Though I think you're reading too much into that tweet. It's not meant to be a literal claim about sales figures. Most manga artists, even extremely popular ones, get nervous about sales figures when a new volume releases.
Compared to the actual ad for volume 3 that was printed in last month's Cune, which calls it "1、2巻が発売後即大量重版中の話題作"
Which is much closer to an actual material statement on how good its sales are.
So I stand by the statement that by Cune standards it's doing absurdly well, and thus Cune's not going to cancel it anytime soon.