It means that the Japanese government scoops up at least 5% of that amount in tax income every year, which equates to $50 billion they otherwise wouldn't have. I wouldn't even be surprised if the figure is actually closer to $100 billion rather than the aforementioned. If you also add the tax on goods people pay when they purchase stuff with Bitcoin in one of the more than hundred thousand stores there, and more aspects that the crypto market brings with it, then it's blowing up even further.
It definitely wouldn't surprise me. I read an article not that long ago where Japan's GDP would likely book a growth of roughly 1% this year on Bitcoin alone. If we add other cryptos as well, we might be close to 1.5%.
This is while the entire crypto sphere is still in its very early stage. If people get paid in Bitcoin, spend their coins directly in stores, this number will only keep growing. Bitcoin boosts Japan's economy, and Japan Bitcoin's utility.
I think that's a pretty fair deal working out well for both parties. I would however like to see BitFlyer release some merchant sales data, because that would grant us a look into how willing people in Japan are to spend their coins.