Well well well, what do you know.
Compression tester from ebay is faulty

Gets to about 50 psi and just gives up. My guess is that the crappy adapter for the smaller spark plugs is leaking, as it looks pretty sh!t quality

Have sent the seller a message asking for a replacement for a working one or a refund. So for now, all on hold.
Car runs a bit flat, seems to lack some power. For now it's OK, but I will re-do the test when I've got a working tester.