“However, the effects produced by a nuclear explosion were not fully realized until the “Johnston Island Test” in 1962. Which date of realization seems to conflict with the finding of the the Teak test performed during Operation Hardtack.
Which caused communications impairment over a widespread area in the Pacific basin. This was due to the injection of a large quantity of fission debris into the ionosphere. The debris prevented normal ionospheric reflection of high-frequency (HF) radio waves back towards Earth, which disrupted most long-distance HF radio communications. The nuclear detonation occurred at 1050 UTC on 1 August 1958 (which was 11:50 p.m., Johnston Island local time, on 31 July 1958)
The detonation spread a layer of fission debris in the upper atmosphere and destroyed the ability of the normally ionized layers of the upper atmosphere to bend radio waves back to the Earth, thus cutting many trans-Pacific high-frequency communications circuits. This blackout lasted 9 hours in Australia and at least 2 hours in Hawaii. Honolulu telephone service was apparently not affected; the Honolulu police registered over 1,000 extra calls that night as startled residents asked for information on what they had seen.
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Which date of realization seems to conflict with the finding of the the Teak test performed during Operation Hardtack."
That's because the shot reported on by the Marine Corps College was probably the Starfish Prime shot. There were several different tests between 1958 and 1962.
Hardtack Teak was done at only 76.8 kilometers, well below the F layer of the ionosphere. It only temporarily blocked waves between the ground and the F(2) Layer (re. HF communications) and was blocked by atmosphere to the sides of it from causing damage over a wide area. Our atmosphere also cleans up debris messes fairly quickly.
The Starfish Prime shot was done at an altitude of about 400 kilometers (about 250 miles), well above the F layer, and left an artificial radiation belt hanging there for months.
The information is easy to find from quite a few sources.
https://en.wikipedia.org/wiki/Operation_Fishbowl#Starfish_Prime
Shots conducted at lower altitudes do less lateral damage because of lateral atmospheric obstruction. Shots conducted at far higher altitudes would cause damage for far greater lateral distances, because the pulses would approach wider areas of the upper atmosphere from above.
So yes, high altitude shots conducted by Iran in advance of strikes closer to the ground would cause lasting damage to communications equipment and other equipment (including damage from radiation at much higher frequencies to computer equipment in vehicles, satellites, etc.). Much of the damage from EMP strikes would be very different from what might be caused by a strong coronal mass ejection from the sun (more of a long wave problem on the ground).
The atmosphere is not very thick for very far above our heads. One tenuously related way to find out is to travel to a location somewhere above 3,000 meters elevation (or above 9,000 feet), try to go jogging and stay out in the sun for a day. Somewhat tenuous, but it makes the point to folks from the lowlands.