Hate to be that guy but I live in Japan and have been trying to follow this news myself. As far as I remember, this guy was just advising that it be used, however, trials for it had not yet been completed, and so far I haven't heard anything about ivermectin actually being prescribed yet. A quick search for "イベルメクチン” (ivermectin) doesn't seem to reveal all that much.

The source in the OP doesn't seem to say anything other than that the chairman recommended it. While it's true that cases have started plummeting, I haven't seen evidence that it's because of actual prescriptions and treatments for ivermectin (but I would love that to be the case!).

My suspicion so far is that test numbers dropped off significantly after the Olympics, where they were no doubt testing FAR more people than usual. In Japan you (mostly) can't just walk up and get a test for free. It has to either be provided by your workplace/school/whatever, or you have to have had symptoms for at least a few days and then be under the supervision of the public health system. You CAN pay money to get a PCR test but it's like 22,000 yen (let's say $200 or something) so people aren't doing that unless they absolutely have to (i.e. need to travel or do something that requires a negative test).

Anyway if there's something that's slipped by me I'd love to be corrected. Last I heard re non-vaxx treatments in Japan was this, a story about a new treatment called Sotrobimab or something, which is similar to the other, already approved monoclonal antibodies treatment.