Image you held 3X GDP in JPY denominated 0% 15 year notes...
(I couldn't easily find stats on the term of the debt.)
If rates rise to 5% those existing notes are only worth about half what they were before. So theoretically raising rates is supposed to increase the value of a currency, but when the central bank holds most of the outstanding debt, it means that the central bank takes a huge, instant hit to its balance sheet.
Then you gave the issue where a huge chunk of their stock market is also government held and you'll have a similar hit to the value of stocks.
Finally, the government is still running a deficit. Borrowing at 5% for that is going to hurt.
I suppose they have a sizeable amount of us treasuries, they could sell those and buy yen. Short term bump. Still if I was trying to value the yen, I would rather see them hold that positive yielding USD debt than 0% JPY debt or JPY cash.
I must be missing something because I never thought they would get away with keep rates at zero for so long in the first place.