That's exactly what they did:
The group simulated the vent environment in the laboratory, immersing various types of clay in pressurized water at 300 °C for several weeks and looking at the fate of a simple organic compound, methanol, in this stew.
Sorry, but it DEMONSTRATES nothing.
It just CLAIMS that it does.
I'm not sure I understand this comment - are you suggesting the experiment is a hoax? (The journal Geology has a pretty impeccable reputation and high standard for publication; I'd hate to think they have their guard down..)
They are ASSUMING that vents were present on early Earth; aren't they?
Sea floor spreading needs a FLOOR to spread and according to the Pangea theory, there was NO floor until it broke apart into our continents.
At that time there were already fossils imbedded in the rock layers,; weren't they?
No, I don't think so, but he might be suggesting hyperbole is involved in the description. It might help to know what "complex" chemicals are involved. I seems certain that they are not above the level of the amino acids created by Miller-Urey otherwise it would be trumpeted to high heaven. Plus I can certify that complex "goo" can be achieved in minutes in the lab using very simple carbon compounds.