Why would you be shocked by flying at heights comparable to typical jet aircraft flying similar distances?
(Aside: the advertised range seems like drastic overkill for something designed to offer a cheap alternative to private jets--it's difficult to overstate how much it sucks being cooped up in a tiny plane for over 11 hours, even if you have the luxury of rich leather seats, can stand up in the cabin, and don't have to pee into plastic bags. Even if a family wants to charter a private jet from LA to the Bahamas for a long weekend, it'd be miserable trying to one-leg it.)
If an airplane like this meant being able to fly private, it's completely worth it. One other factor you have to consider is that this plane can probably use tiny airports which shaves a 4 hours of additional driving and security gates. I'd rather be in a slower flying plane for 7 hours than fly for 4 hours but spend 4 additional hours at security gates and car traffic.
Still, "low cost" is relative to a chartered private jet--which, emphatically, is not a service that people purchase because it's the most economical option--rather than a commercial airliner (although, if true, the SFC per passenger of the Celera would be in the same ballpark as modern commercial airliners, which is very impressive). The average domestic flight is under 1000 statute miles (https://www.bts.gov/content/average-length-haul-domestic-fre...), so this plane needs to offer compelling efficiency gains at ~500-1500 mile ranges in order to compete with existing services, unless they're strictly planning to fill the niche of one-leg, long-range flights that existing private jets don't have the legs for.
$300/hour charter price with 6 seats for a 5 hour trip (2000 miles) is about $250 per seat each way. This is pretty competitive.
[1]https://patentimages.storage.googleapis.com/3b/f3/9a/928c862...
A design cruising altitude of FL500 implies that they're seeing nearly all of their efficiency improvements due to the thin air at altitude (flying around 200 mph indicated, which isn't totally unreasonable for 20 mpg in a small-ish piston-prop), but I doubt they have the climb rate to actually reach that altitude in anything short of a transcontinental flight.
I like to think they crunched the performance numbers once or twice, but if they're off by the same amount they were with their initial SFC calculations, there's no way they'll reach an acceptable cruising altitude during an average flight.
I don't think many flights today are cancelled because of weather on the route there, it's the weather over each airport, which would still impact this aircraft.
Many other six seat planes are like that.
A second factor is the effect of a lifting body on stability, and the cost of compensating for it. Even in the case of nominally non-lifting fuselages, they tend to produce lift at high angles of attack, and that tends to be destabilizing, producing an increasing pitch-up moment as the angle of attack increases (ordinary straight wings do too, but the effect is proportionately less, as the wing chord is considerably less than the fuselage length.) This has to be corrected with measures (typically a larger stablizer) that also create drag. Having a fuselage designed to produce lift exacerbates this problem, commensurably with the amount of lift being generated.
Modern sailplane design is a case study in the persuit of aerodynamic efficiency, at least at low speeds, and lifting bodies have not proved to be useful. The fact is that, once you are flying, you do not need more lift; what you want is less drag.
Cost to repair and insurance is also a very significant part of the cost, so if this is difficult to land or costly to repair, it won't be very popular.
It's optimized differently to the experimentals, and it's more useful commercially. Flying high avoids a lot of weather issues. You don't have to cancel flights because of weather.
The drag from the streamlined shape is very low. It's just a textbook minimum drag shape.
The cruise speed advertised does seem high still.