Using a strategy used by many on the right, it sought to avoid the economic argument by playing the people who wrote the letter, claiming many were not economists. Apparently, for example, a retired economist is not really an economist. Nor is a historian allowed to comment on the subject. And heaven forbid someone working for a pressure group be given the title 'economist': that would never do.
The trouble is that this is a particularly foolish argument, and there's ample proof of this. In particular, Adam Smith was not, using this definition promulgated by the institute that abuses his good name, an economist. He was, after all, a professor of moral philosophy and that, quite clearly, using the ASI methodology, would not qualify him to comment on economics.
If this is the best the right can do their intellectual arsenal really is bankrupt.