Money is debt and shows up on the balance sheet as a liability. A T bill is an asset. An asset can be borrowed against. These are just hypothetical numbers. You buy $100,000k worth of BoJ treasuries, now you have $100,000 in assets at 0% interest. Now you use those as collateral for a loan...say 10 to 1 and buy $1mil of US T bills at 4%. If your interest rate is 2%, you net 2% interest on $1mil. And now because you have $1mil in US T bills (assets), you can borrow against that.

Where this blows up is when the BoJ interest rates rise and make that trade not profitable. This is similar to how Microstrategy buys bitcoin. If they can get cheap money and bitcoin keeps going up, they can keep leveraging and buying. If bitcoin falls or they can't get a favorable loan, the whole company implodes. They're already leveraged to the tits via loans based on the future value of their stock price, which is collapsing. If they're forced to sell bitcoin, it's not going to the moon, it's going to hell. Unlike something physical like metals or a company, there's no underlaying asset to fall back on. A bankrupt business will still have value left from real estate, equipment, patents, etc. Bitcoin is 1s and 0s.