AlphaBay is now behind DreamMarket. And DM will not implement XMR they stated. In their Forum also no one asks for that. I did some research and AB once claimed they did it cause their customers demand it. I looked through the whole board but I couldn't find any demand for that. Somehow I think the group who said AB just used this opportunity to pump and dump is probably right.
First the reddit said it is not planned... this doesnt mean it will never happen.
Second the report where you got this nice little picture from also has this in it
Quote
Another report by the Department of War Studies at Kings College London earlier this year concluded that Bitcoin is still the most popular cryptocurrency on the dark web. Interestingly, the current case study notes that other cryptocurrencies are gaining more users in dark markets, such as Monero.