Yep, he used an ammonium fluoride buffer. > Instead of a standard HF etch, a buffered oxide etch of NH4F (Ammonium Fluoride) in HF can be used to control the etch rate and photoresist lifting. I use approximately 20-30g of 100% NH4F per 50mL of HF (stock whink rust remover)

Ammonium fluoride definitely isn’t as easily accessible as rust cleaner, but you could buy it for a somewhat cheap price on Amazon.