A fairly well known cavalier breeder offers this helpful information on SM on her website:
How extraordinary -- when every research group in this area has stated a minimum of close to 40% of all breed samples are affected *with actual syrinxes*.Implications in Cavalier King Charles Spaniels
"I have a Cavalier, should I be concerned"?
This particular condition is known to affect less than 0.002% of Cavalier King Charles Spaniels so odds are very much in your favour that your dog would not have this condition.
Perhaps she defines 'affected' in a different way than the entire research community? The words 'head' and 'sand' come to mind.
I don't think there's a club left that believes the level of affectedness is this low.
This reminds me of the sites that still claim no MVD in their colours or lines. I had hoped show breeders had moved beyond this kind of claim.
Conclusion: if you are buying a puppy be sure to talk to your chosen breeder about this health issue and make sure you are dealing with someone who doesn't pretend like it isn't a concern. It is, as is very clear from the openness of reputable breeders to discuss it just as they discuss MVD. The chances remain low of seeing a symptomatic dog but all good breeders recognise that SM is a serious problem and that the goal must be to keep the number of symptomatic cases as low as possible.