The reason, of course, lies in the text of the letter. (Disclosure: I am friends with two of the signers and have an immense amount of respect for their work.) Before I explain, here is the text:
We believe the Federal Reserve’s large-scale asset purchase plan (so-called “quantitative easing”) should be reconsidered and discontinued. We do not believe such a plan is necessary or advisable under current circumstances. The planned asset purchases risk currency debasement and inflation, and we do not think they will achieve the Fed’s objective of promoting employment.
We subscribe to your statement in the Washington Post on November 4 that “the Federal Reserve cannot solve all the economy’s problems on its own.” In this case, we think improvements in tax, spending and regulatory policies must take precedence in a national growth program, not further monetary stimulus.
We disagree with the view that inflation needs to be pushed higher, and worry that another round of asset purchases, with interest rates still near zero over a year into the recovery, will distort financial markets and greatly complicate future Fed efforts to normalize monetary policy.
The Fed’s purchase program has also met broad opposition from other central banks and we share their concerns that quantitative easing by the Fed is neither warranted nor helpful in addressing either U.S. or global economic problems.
The reason lies in the first paragraph. “The planned asset purchases risk currency debasement and inflation, and we do not think they will achieve the Fed’s objective of promoting employment.”
First, they said only that QE risked inflation, not that it would cause it.
Second, they said that QE would fail to “achieve the Fed’s objective of promoting employment.” That certainly does seem to be the case, no?
The right question for the signers, I think, would be whether the failure of inflation to appear has caused them to reconsider their opinion about the efficacy of quantitative easing. But it is true that the wording of that sentence allows the signers to truthfully claim that they thought the policy ran risks with no hope of reward, which is a different thing from saying that they predicted rampant inflation in the letter.