Quotes (single or double) truly are not necessary to search an entire phrase, such as
the words I have bolded here. Search for those words in "exact" mode (sans quotes) and you should get this topic in your results.
However, some words may be assisted by the quote marks
IF AND ONLY IF there are actually occurences of the entire string,
quotes included. In the case of the 'AT' command, this is true--in fact, this post is one of several in this topic that contains the string "'AT'".
Admittedly, the search page does not handle the situation well if you give it too big a mouthful to chew. This is an unfortunate limitation of computers: they simply refuse to budge if their brains are not up to the task--whereas I've known quite a few adult (and teenage) humans who appear to function normally even when their brains take an extended leave of absence. :D