It's a load of crud. Look at this section, ostensibly about whether the building was designed to withstand a jet impact:
2. Withstanding Jet Impact
FEMA: “The WTC towers had been designed to withstand the accidental impact of a Boeing 707 seeking to land at a nearby airport…” [2]
...
Makes sense, right? They even admit that the operative words are 707 and 'seeking to land'. Then they insert this quote from Skilling taken from a Seattle newspaper in 1993:
Concerned because of a case where an airplane hit the Empire State Building [which did not collapse], Skilling's people did an analysis that showed the towers would withstand the impact of a Boeing 707.
Again, makes sense. But then they go off on a tangent by continuing the quote from the Seattle article:
Skilling - a recognized expert in tall buildings - doesn't think a single 200-pound car bomb would topple or do major structural damage to a Trade Center tower. The supporting columns are closely spaced and even if several were disabled, the others would carry the load.
…Although Skilling is not an explosives expert, he says there are people who do know enough
about building demolition to bring a structure like the Trade Center down.
"I would imagine that if you took the top expert in that type of work and gave him the assignment of bringing these buildings down with explosives, I would bet that he could do it."
This has nothing to do with whether or not the WTC could withstand the impact of an airline. It merely shows that Skilling believed in 1993 that some hypothetical person could, if given sufficient resources and time, demolish the building with expolosives; without suggesting whether or not Skilling had ever actually researched the controlled demolotion of the towers (and why would he, when they weren't planning to demolish them). Does it have anything to do with whether the building was designed to withstand a plane crash? Nothing whatsoever. Does it have any relevance to the article, at all? No. How did this get past peer review? But it gets worse as the 'letter continues'
Thus, Skilling’s team showed that a commercial jet would not bring down a WTC Tower, just as the Empire State Building did not collapse when hit by an airplane, and he explained that a demolition expert using explosives could demolish the buildings. We find we are in agreement.
The Empire State Building is completely irrelevant. It is a different building and a different plane travelling at a different velocity etc. Their conclusion is also completely wrong, the letter is supposed to be about where they agree with FEMA and NIST:
Our goal here is to set a foundation for scientific discussion by enumerating those areas where we find agreement with NIST and FEMA.
Here they are not agreeing with NIST or FEMA, instead they are agreeing with an 1993 article from a Seattle newspaper (actually a straw man argument they have created from the newspaper article). It appears that they are either dishonestly trying to suggest that NIST or FEMA have suggested that the WTC should have withstood an impact or they have no idea what they have written.
I suspect that they might have used the loophole that lets you nominate your peer reviewers to get around the, apparently minimal, scrutinisation process.
All I can suggest is that the authors were forced to pay $600 to publish their letter there because it would have been torn to shreds if it was published in these forums.