That is correct, any adapter source that purports to support QC3 via a C-connector does not comply with USB spec. No legitimate manufacturer is going to sell such a product.
Why Xtar chose to employ a C-connector on the VC8, and not a micro-USB connector like they did with the VC4S is puzzling, and a question only they can answer. Especially when the VC8 does not support USB-PD input. There is really no good reason to do so, except to make an hollow boast about having a C-connector, and put users like yourself into confusing situations. That is not unlike when lights first appeared with USB-C ports, but could only charge via A2C cables. Good for show, not for function.
QC4 and later are compatible with USB-PD, and do employ the C-connector.
Since the newer standards are made to be backward compatible, a QC4 (preferably 4+) adapter is a possible solution, if you wish to standardize on C2C cables. A QC4 adapter, being PD compatible, will also charge your iPhone and iPad at full speed, but not your MacBook Pro, since it is limited to 27W maximum in PD mode. For that, you should stick with the RavPower 91W adapter you already have on hand. QuickCharge is a proprietary standard developed by Qualcomm. Apple has never employed QC in any product, only its Apple 2.4 standard, and USB-PD on newer products.
Otherwise, you’ll have to rely on an A2C cable to supply the VC8 from a QC3 adapter.