I believe the number is correct. The source from the government: https://www.mlit.go.jp/tagengo-db/common/001556845.pdf

I assume you've never been there. 一ノ倉沢 is really impressive and dangerous.

I've climbed routes in ichinokura a few times. It is impressive, and some of them are quite poorly protected. But 800 climbers seems like an exceedingly high number, and that report seems quite vague and unsubstantiated, even if it does come from the government. So I remain sceptical.