"Lossless" just means there is no (lossy perceptual) compression involved. You could absolutely have a 48khz lossless signal, or even 44.1khz if you were dealing with a CD rip, and it would likely still measure better than the lossy equivalent audio sent over AAC or aptX via Bluetooth.

I'll skip the whole debate about how good perceptual lossy encodings actually are these days and whether any person could reasonably expect to hear that measured difference, but just wanted to clarify that the sample rate of the headphones' built-in ADC shouldn't be conflated with the lossless codec.