Because they have multiple different bonds outstanding with differing maturity dates...failure to pay back one is indicative that there may be successive failures after this one.

From other articles the issue is that they can't even pay a "meme NFT" ...

If they can't pay that, what can they pay on their 300B in other debts?

Other news sources indicate they don't have money coming in, possibly NONE, the real estate market in China is getting cold... and they have unfinished projects that they already collected money on that require money to complete.

Because they actually are in debt to the tune of $300bn but they've managed to keep playing so they stay afloat. This would be the moment the music stops.

The payment on the debt was due today, and as of 3 hours ago that timeframe has passed, which is why the default got announced by the DMSA.

Thats not the size of what they owe, its the size of what they owe today.

When you have 0 liquid dollars then $148 million is more than you have.

Because they don't have it

This is interest payment.

They need to pay that in USD not USDT. One of those can be printed at will by your NFT wash trading buddies. The other is not so easy to get ahold of.

“large company” doesn’t imply solvency