OK 17% is not a small proportion. That's true. But I wouldn't say the letter tries to brush it under the rug. Do you mean this represents a climb down from their original position? That's probably right.
They say explicitly that
1.
they have given up trying to define pure beer and
2.
don't apply a Reinheitsgebot type prescription that it can only contain malted barley, water, hops and yeast.
And they close by saying
3.
that what matters for counting as real ale is the conditioning and dispensing arrangements.