I think it's not so much that they don't have liquid assets they can sell.

The issue is those liquid assets are US Treasuries and US public market equities (mag7 etc.).

They don't really want to sell them, and they also know that the US really doesn't want them to sell them - the last thing Trump wants heading into the midterms is an S&P500 bear market and 10y treasuries heading back to 5+%.

So they ask for a swap line and they're negotiating from a position of strength, the US doesn't have much of a choice but to give them as much as they need and damn the consequences