I'm the one who suggested that the cylinders were washed. lol.
The piston rings seal with a very thin coat of oil on the cylinder wall. If you ever get fuel without starting, like trying to get a bunch of old gas and a dirty carb to run, the fuel washes this thin oil coat off the cylinder walls, and you lose compression. Fixing the problem that caused it not to start (i.e. flushing bad fuel, and dismantling and cleaning a gummed up carb), and starting it, will get oil slung back onto the cylinder walls, restoring the ring seal, and restoring compression.
Sticky valves or such aren't going to happen simultaneously to all four cylinders from a running engine... Mechanical things that could affect all cylinders at once are a broken crank, broken cam (I saw this once! ... but there was a lot more broken too), broken timing gears, slipping ring gear (so you're not actually cranking the engine), etc... But if the accessories and dist are turning, those are all going to be fine. Broken cam or timing gears also cause a distinctly different sound, since some cylinders will be stuck with both valves closed, and compress each revolution - the sound of an EJ ready for a timing belt and half the valves replaced....
Am I positive I'm right? Of course not. They could have blown both headgaskets at once, or some other simultaneous multiple failure. Or the engine is total crap and never actually had any approximation of full compression but was limping along. But, based on my experience, I think it's the most likely diagnosis, which is why I said "probably".