Do you believe that he is honestly mistaken that Trump's actions are not impeachable? Or do you perhaps believe that he's right and Trump's actions are not actually impeachable?
The longer answer:
You first have to define "impeachable offense". Justice Rhenquist famously wrote a book about impeachment before presiding over the trial of Bill Clinton. His conclusion was that impeachment was a purely political process. What that means is that literally any act is an impeachable offense of 67 Senators say it is an impeachable offense. As the saying goes, you can impeach the president for eating a ham sandwich if you have the votes to do it, and I think Senator Alexander knows that.
However, the Constitution calls for impeachment in the case of "high crimes and misdemeanors". That has led many to conclude that there has to be an actual crime to impeach. I thought that myself a few months ago, but a very small amount of reading convinced me otherwise, and pwengthold's example of a president not doing the job of being president is an excellent counterexample that everyone can understand. (It was pwengthold, wasn't it?) Nevertheless, some people insist there has to be an actual crime. Is Alexander one of them? If so, he's wrong.
Then again, if it doesn't have to be a crime, what could it be? Are we back to ham sandwiches?
I think the answer is that any abuse of power should be considered what I might term potentially impeachable. In other words, you could impeach for it, but you don't have to. If the action isn't very serious, then it might be an abuse of power, but not sufficiently severe to warrant removal from office. Every single president in my lifetime (my memory begins with Nixon) has been accused of abuse of power. All of them. Some of them with quite a bit of merit. Should they all be impeached? Could they all be impeached? How do we decide how severe an abuse of power has to be before saying that the President has to be removed, without waiting for an election, and even if he would win that election. (Maybe especially if he would win that election.) When should the option be removed from the voters?
There isn't a set of lawbooks we can turn to to make that call. It really is a matter of judgement, of opinion. How bad is it? Is it bad enough? In my opinion, it has to be pretty darned bad before we disregard the voters' will, or before we refuse to allow the voters to speak. That whole election process is a really big deal. I mean a really big deal. It's a defining characteristic of America. You don't lightly set it aside.
So, is this particular issue big enough? It's a judgement call, and I won't say Lamar Alexander is mistaken in making the judgement he did. Neither will I say that the Democratic Senators who voted to remove him (or will vote on Wednesday, anyway) are mistaken. I don't think there is a right or wrong answer. I don't think the situation is clear.
So, in summary, if Senator Alexander thinks there absolutely has to be a crime, I think he is mistaken. On the other hand, if he thinks it should only be a very serious offense or abuse of power, but not necessarily a crime, I would agree. As for whether this situation is sufficiently severe to warrant the very severe step of removing an elected president form office, I think reasonable people could differ.