I've searched, but I couldn't find much information about his level of Japanese.
The only thing I found was a supposed interview he did with Benny Lewis.
https://s3.amazonaws.com/Fi3mplus/damien.doc
By then, he had lived in Japan for at least six years, and the idea behind Anki was to improve his Japanese.
He also said the following, assuming the interview really happened:
"I always told myself that until I have become a master of Japanese, I do not want to study anything else. I'm not sure when exactly that point will come. Yeah, personally SRS has had a huge impact on my own studies. And once I discovered them my Japanese shot up a lot. I've reached the point where I can be pretty comfortable in daily life in Japan. Well, I'm not in any way near master level but I was able to really accelerate my performance."
It has been 18 years, 9 months and 12 days since Anki was developed. That is plenty of time to reach L1 or near-L1 level in any language, so how is his Japanese now?
I'm very curious about this because I'm pretty convinced about ALG statements of manual learning (which includes the activities done with flashcards, like active recall and forcing speaking even if it's just in your mind) causing permanent damage (thus making L1 level impossible and near-L1 level basically impossible too: https://mandarinfromscratch.wordpress.com/automatic-language-growth/ ). As the creator of Anki AND a motivated learner who lived in the target language's country, he should be a good representation of the long-term effects of flashcards (paired-associate learning in general) on language acquisition/growth.
by Quick_Rain_4125