> I think that in that case, if I delete the cookies, the account will log out,
The cookies are deleted when you log out. Leaving your session logged in is what causes the risk of session hijacking.
On my tablet device if I log into freebitco.in then switch to another app then come back to the browser app I find I have been logged out and have to log back in (including 2FA) which doesn't happen on the PC.
Most strange.