Apple couldn't get the free-placement aspect of AirPower to work to its satisfaction. Without it as a distinguishing feature, it would be hard to distance itself from other brand name chargers, and the multitudes of cheap chargers on Amazon.
iOS 13.1 and later requires a charger that conforms to the Extended Power Profile to charge at 7.5W.
For products in this price segment, the MFi-certified puck is built-in, and its cost factored into the price. The cheap BYOP chargers are cheap because they don't.
Although the SliceCharge does include an MFi watch puck, the main charger is not Qi-certified. Considering that the cost of a WPC membership is as little as $5000, one has to wonder why HardCiderLabs has not joined, and submitted their product for certification testing, when it makes a big point of the watch puck being MFi-certified.
Many a cheap Amazon wireless charger are Qi-certified, which brings questions to mind why a premium-priced product such the SliceCharge is not. If it's not a question of cost, then that suggests that it either does not entirely comply with the Qi standards, or does not pass testing. The AirUnleashed is another example of a not-inexpensive, but non-certified charger.
Personally, if I'm paying ~$100 or more for a product, I'd expect it to meet the standards that Apple expects, and can and may enforce at some point, in addition to all the other expectations in the high-end segment.
13.1 crippled a lot of BPP chargers that employed a kludge to increase charging speed. Not such a big deal for a charger that cost 20 bucks. 100+ bucks, OTOH…