The failure mode of a bolt-on booking engine is that it sells from its own copy of availability. The property discovers the mismatch when two guests arrive for one room.
A public booking request here goes through the same pooled sellability guard as a booking keyed at the desk. It sees group blocks, allotment ceilings, stop-sells and length-of-stay rules, because there is one guard and every sale path routes through it.
The public path also keeps the floors the admin path can waive. An operator may backdate a stay that already happened; a guest may never book the past, and the default that prevents it is on unless a caller explicitly opts out.
What lands is an ordinary reservation on the ordinary board — same status trail, same deposit handling, same folio.