Again, read and try to understand what I said. I've tried to get a handle on this, and what I'm about to say is my current understanding of the issue.
I claimed little or no danger from the horizontal component of the well. I did not claim that legacy wells are no danger at all. They clearly are. Hard to dispute a 30 foot geyser from a legacy well.
All wells begin at the surface, and must pass through surface layers. A well is vertical for 5000-9000 ft, and then turns horizontal. That horizontal portion is well below the depth of any legacy wells, and that pressure will not escape upwards.
The well casing, provided it's done correctly, only goes down to below the surface water table, i.e. the depth where gas and fluid can escape to the surface "under normal circumstances". This is typically only a couple of hundred feet deep. Between the bottom of the casing and the horizontal bend are thousands of feet of unprotected, pressurized bore hole.
When pressurized, this VERTICAL unprotected section does indeed pressurize the gaps between rock layers radiating outwards from the well itself. And since this is below where the gas and fluid can escape upward, no problem. Normally. But legacy wells can provide that conduit, effectively making the surface water table deeper than anticipated.
So the author is right as far as legacy wells do create an issue, and that identifying all legacy wells is helpful. Here's the sentence that, while not technically wrong, is misleading:
Even though the Marcellus is quite deep, between 5,000 and 8,000 feet, the extraction of shale gas through horizontal drilling and hydraulic fracturing or fracking, a process that involves injecting huge amounts of chemically-laced water under high pressure to blast the rock underground, was displacing gas in shallower geologic layers, where old drilling holes acted as natural pathways all the way to the surface.
Nothing he said was "wrong", as the "process" does displace gas in shallower geologic layers, and old drilling holes are acting as natural pathways to the surface. But the fact that the wells are horizontal has nothing to do with it, and the displaced gas isn't coming from 5000-8000 ft deep.