It's a good example though of the benefits of having a Letters Page in a magazine. Not only do letters offer readers a chance to comment and feedback upon the articles and the magazine, but it also gives the editor an opportunity to offer a right of reply.
The letter in question queries the writer's statements about the advice offered to fellow photographers, when the photographs used to illustrate the article did not follow the advice the writer was giving. It's a valid point for a reader to make. If article writers make a statement and enclose photographs, the photographs should at least back up the article. (It's no good saying that the beach offers an opportunity to escape from the crowds, if the photo of the beach demonstrates there's not room for an ant to move!)
However, because the letter writer made this point, disputing the article, the editor has used the Letters Page to clarify things. Firstly, the editor has commented, "The original text that [the writer] Ian provided for the article was edited to fit available space, but that edit did change the understanding of the original text." (Note: this doesn't mean to say that the writer didn't produce an article of the right length - he may have been commissioned for X number of words, and provided them, but a change in the magazine's contents could have reduced the space available for his piece, hence the need for the editor to start editing.) The editor then allowed the article writer to add their comments to, offering them the right to reply, too.
So, if you're targeting a Letters Page, don't automatically assume that you have to be gushing praise in order to get published. If you have a valid criticism about something you've read, your letter could give the editor an opportunity explain, apologise or to clarify. You don't have to be praiseworthy to be published, when writing letters. If you're criticising an article and raising a valid point, it certainly proves that you've read the magazine!
And if you write articles, just bear in mind that the Letters Page could offer you the opportunity to put things right. A similar situation happened to me once, where an editor was forced to cut the space available and had to edit my text, changing the meaning. They apologised on the Letters Page and gave me a few words to say something on the matter too!