He's still been the most successful investor in modern history, including covid. It's a blip to him - and he's right. He's one of the few to consistently beat the S&P 500, and even so, he recommends just buying the index.
The performance of the Nikkei is significantly more complicated than just a low interest rate. For example. the lack of productivity and population growth in Japan is another set of pieces that also impacts the rate of return.