IIUC, they ignore it because they're supposed to. If the market collapses I think Meta can technically just walk away and they lose access to those data center

IIUC, they ignore it because they're supposed to. If the market collapses I think Meta can technically just walk away and they lose access to those data centers (which they no longer want anyways) and the SPV is stuck holding $X of assets with $>X liabilities and the issues of the credit are on the hook but not Meta. And investors are fine being on the hook because they get a higher return from the SPV bonds than Meta bonds. (risk adjusted it's probably the same return).