Lewis provided no opinion about how to read the books the first time, other than his publishing order. He thought rereading in internal chronology was acceptable, though.
Lewis's letter to Lawrence Krieg needs to be read in the context of what Krieg's question was.
Per NarniaWeb, Krieg was:
preparing to read the Narnia books for a second time. Lawrence
wondered if he should re-read them chronologically, but his mother
felt he should stick with the original published order.
Lewis' response:
I think I agree with your order {i.e. chronological} for reading the
books more than with your mother's. The series was not planned
beforehand as she thinks. When I wrote The Lion I did not know I was
going to write any more. Then I wrote P. Caspian as a sequel and still
didn't think there would be any more, and when I had done The Voyage I
felt quite sure it would be the last. But I found as I was wrong. So
perhaps it does not matter very much in which order anyone read them.
I'm not even sure that all the others were written in the same order
in which they were published.
therefore is talking about the proper order in which to reread the books, not to read them for the first time. Lewis apparently felt that rereading the books in internal chronological order would be interesting, but it was not a strong preference. I have seen no statement by Lewis about the order in which to read the books for the first time - but surely the original order he published them is a strong clue as to his real preferences in that regard.