hmm... just a guess here. Maybe he means alterations or other extensions.

I would suspect that he suggesting 1-3-5-b7-9-11 which is the 11 chord. Some might play a suspension chord which eliminates the 3rd of the chord which is incorrect for being pure.

A9 sus4

Since we cannot play all 6 notes in our fretting hand, we need to look at the notes that truly make up the chord 1-3-b7-11.

Please note that these are all close voicings.

A11

A11

A11

Thanks for the replies. I think the problem with the 11th chord is that there is a very dissonant sound when the third is played as it is just a halftone away from the 11th (or 4th). I guess that would make the interval a minor 9th, e.g. in C the interval from the E to the F.
I've examined some of Joe Pass' chord melodies and it seems that he usually omits the third when he plays an 11th chord.
Perhaps this is what he means when he says that the 11th has to be played pure.
I'm trying to reach an old friend who studied guitar formally at Berkely to get his take on the question.

Tony - Please note that when you omit the 3rd it becomes a suspension chord. For a pure 11th, the 4th needs to be over an octave away. This way no dissonance between the 3rd and 4th occurs.

There certainly is a dissonance when the 4th is an octave above the 3rd (minor 9th to be precise). This isn't improved by raising the 4th another octave. It's improved by putting the 3rd above the 4th, making a major 7th between the two; or of course by omitting the 3rd altogether, to create a sus chord as you say.
Normally a jazz player will interpret an "11" symbol as shorthand for a 9sus4. If they do decide to include the 3rd, they would put it above the 4th.