What if you lay it upside down?
Horizontally right side up has the APU facing up, so that plastic or silicone seal around the APU would be very tight and reliable. Upside down would rely way more on the foam seal against the heatsink base plate, but it's still very unlikely that the liquid metal would leak out sideways. I'd guess they likely tested upside down as well just in case some people didn't use the stand and had it upside down by accident or on purpose. I don't remember who but one of the PS top brass posted a pic of their PS5 upside down prior to launch or not long afterwards.
Vertical is the only orientation that would look to be possibly problematic from the get go. Sony would've known this and would have focused on testing this positioning more so than horizontal. It's hard to believe they missed something while testing, especially since we're talking about a conductive liquid in an electronic device. How much they may have changed the liquid metal formula or seal design or its materials over the last 3 models is something that could be questioned. Yet Sony would've done the same testing again for those changes and they obviously passed for production.
The thing about testing is you can never test for everything because sometimes (unexpected) shit just happens and there's no limits to play or stupidity. I'm with everyone else in thinking this is just like the Series X overheating and melting the case or blowing smoke out the top fan. All made up or just a few of the unfortunate rare occurrences that happen to any device.