The kind of mortgage-backed securities they held are still US Government securities! They are “mortgage-backed” more-or-less in the sense that the Government is willing to offer such-and-such amount of investment opportunity at so-and-so rate of return because they are holding a bunch of mortgages that they expect will be able to pay for it. They are different to Treasury bonds in important ways so it’s imprecise for me to refer to all of it as Treasury bonds, I agree.

In practical terms, the reason SVB held MBSes was the same reason they held Treasury bonds (low risk because of Government guarantee), Treasury bonds and MBSes react to Federal Reserve rate increases in much the same way, and the ways that MBSes and Treasury bonds differ are largely walled off by the Government guarantee and thus immaterial to SVB (if the housing market does some wild stuff then MBSes might pay out differently to Treasury bonds, but this didn’t happen here).