Where have you seen that? As far as I've heard airside transit has always been fine, except that you can't actually enter the country which means no airport changes and no overnight connections at NRT (which closes at night).
Even if you do want to buy an onward ticket to somewhere else you'll have to wait and see what countries you can actually enter and find flights too as nobody knows yet what all the different restrictions will be like in March.
The problem will be that the airline taking OP to Japan will deny boarding as he is ineligible to enter the country. This is the problem with separate tickets.