I’m a vet tech and to me the symptoms you described sounds like a respiratory infection. I think the doctor suspected the snake to have a RI and not a URI; this is why she did a lung flush. I have no idea why she did not do a culture/cytology. Doing this would give the doctor a better idea of what kind of antibiotic/medication are need to treat the patient for a bacterial, mycoplasma, mycotic, viral, or parasitic pneumonia. The doctor is doing the trachea or lung wash not only to clear out some of the discharge, but also to collect a sterile sample of the mucus/discharge.
In our practice the sample taken is set up for an in-house cytology for the doctor to look at and the rest is sent out to a lab as a culture. Most of the time the doctor will give an antibiotic injection afterwards and send the patient home with oral or inj. antibiotics. Depending on the results once the cytology is sent back the doctor would either keep the patient on the meds or sometimes a different medication or antibiotic is needed.
I don’t know why your vet did not explain this to you, or why they dealt with this case in the manner they did. Before finding another vet I would express your concerns, and disappointments to your vet, especially if the symptoms have no improvement. Trust me sometimes a clients concern is all that’s needed to give doctor/techs a wakeup call and help them get back on track. If conversation does not result in better pet health care than I would advise finding another vet. And remember in human and pet care all doctors are "practicing medicine" and just because the treatment did not work the first time around doesn’t mean it’s the doctors fault. Hope this helped.