I think the "don't use foldable seats" is a generalisation based mainly on forward facing seats. The fact is that rear facing, assuming the seat passes any kind of regulatory crash test, offers better protection than even the best forward facing seat. So presumably, even a foldable rear facing seat is safer than a single-piece forward facing seat. The safety isn't coming from the features of the seat itself, it's coming from the fact it's rear facing (if that makes sense).
In terms of independent crash tests, this one hasn't been through any so can't say how it would perform in those yet. I did hear talk about plus testing but it seems to have all magically vanished from their website - possibly they could not get it to pass that. Carseat.se reckons it's done an "unofficial ADAC test" and did well but I honestly don't know what that means - take with a pinch of salt TBH.
I would not say it's "minimally tested" though - this is a bit of a meaningless phrase for a start, but also it suggests that the company is aiming for the bare minimum to pass the test, and I do not get this impression with this company. As I said - not seen a crash test or any published numbers, and a portable seat is likely to have to make some compromises in comparison to a fixed/one-piece seat - but there are some things which do tell you this company is looking at safety:
The seat has a support leg which is positive. Support legs make a big difference to rear facing crash safety. The rear facing up to 125cm is impressive as is the weight limit of 23kg - these are similar to Axkid One2 and Joie i-Prodigi. Axkid One2 is probably one of the safest seats in the world due to the combination of long rear facing/Swedish plus test, and ease of use.
The main sticking point with this one really is the price. And honestly it's not that light - it's about 10kg - it is less bulky though than the more basic/seatbelt fitted "standard" car seats.
In terms of travel/foldable seats, you have three rear facing options which would fit an average 3yo - Tinyseats original, Tinyseats 2 and Nachfolger Hy5.
Depending on the size of your 3yo, you might be able to use Tinyseats original or Nachfolger, but they won't be suitable for as long as the Tinyseats 2.
You could also look at forward facing seats - they are less protective than rear facing but the difference is not as huge at 3. However, you do get into the conundrum that the portable/foldable ones aren't going to be as robust as a standard/fixed one piece shell, and when you're forward facing, the quality/features of the seat matter a lot more. I also think that with these seats (Urban Kanga/Maxi Cosi Nomad) you are prone to getting forward anchorage because the belt path is extremely shallow.
You could look at some of the travel high back boosters, like Chicco Fold and Go, as these are advised minimum age 3. However, these just use a seatbelt, no internal straps and you might not be comfortable with this at age 3. Also, some 3yos will not meet the minimum weight and height requirements (15kg/100cm) and there is some good evidence from the US that 15kg is too low as a minimum for a HBB seat, and 18kg is much better in terms of injury reduction. It's not known whether the improvement is due to the child's physical size or the fact that 18kg children tend to be 4+ whereas 15kg is a more common weight at age 3. Most companies are now recommending age 3.5 or 4 or even 4.5 as minimum ages for their high backed boosters and experts back up this recommendation.
All really depends on what you're after - what is the most important thing? Do you NEED it to be a compact seat? A lightweight seat? Universal fit? Is it important for you that it's rear facing? Is your 3yo big or small or average sized? Do you need to use it once or do you want to be able to keep using it in future? Do you want the highest possible safety rating and everything else doesn't matter? Or just for it to be safe enough to be legally approved and offer some protection? Is this a better than nothing scenario, or are you looking to improve on something (e.g. a lower-quality hired seat)?