Closing the thrust levers DOES disconnect the Autothrust.
Disconnecting the Autothrust with the FCU PB will cause the Autothrust to disco but thrust is locked in the setting it was in, until the thrust lever disco buttons are pressed or Autothrust is re-engaged.
Alphafloor is available even if Autothrust is not engaged. It isn’t available f Autothrust has been perm de-activated (>15s on the TLs buttons) or if Autothrust has failed.