But accessing the securities in that account might be delayed if the bank were in receivership -- the FDIC might freeze everything while it sorted out what's what. I don't know how that works.
If the securities are in an account at another institution that certainly wouldn't be a problem.
so if I own $1M in Treasury bills I'll get them back while if I own $1M in cash then I'll get back $250k?
Yes. (I mean, you may even get more. But the important point is that in the first case you do not have a deposit and the bank doesn't owe you anything.)
with some caveats: if you keep $250k in your personal checking, another $250k in your personal savings, another $250K in a joint checking account, another $250K in a joint savings account, all accounts being with SVB, you will get $1M back.
FDIC states "The standard insurance amount is $250,000 per depositor, per insured bank, for each account ownership category"