upon further investigation, this is what the printed terms say about closures:

"Thankyou member account closures: If your thankyou member account is closed for any reason, you will lose any thankyou points in that thankyou member account. if your participating citi account is closed, you will no longer be able to accumulate points and may lose the thankyou points that you earned through that citi account and through thankyou rewards. if all your citi accounts are closed, we may close your thankyou member account. if you do not have an open citi account citi account, we may also close your thankyou member account or suspend your account privilages at any time for any reason".

bold emphasis mine. There is no "30 d" wording... but I guess they could, in theory, erase all the thankyou points you had in your combined member account associated with the citi account that was closed - while letting you keep any points associated with your other open citi account.