I have to take issue with this comment, as I believe it is not an accurate representation of the real message or real reasons why someone may not be a fan of sparring.
Rarely does anyone but the truly delusional make the tired old claim that their method is "too deadly" for sparring. The accusation of that claim is itself a tired old claim.
It is true that some things are not appropriate for sparring. Certain things can be of little effect or even no effect at all if they are not done with full intent and full commitment, which would mean that they are then destructive. Destructive things, done with destructive intent, cannot be done in the context of sparring. I hope I don't need to explain that notion further, for anybody here.
These destructive things can be dialed back for use in sparring, but they are then ineffective. As such, it then becomes the responsibility of the sparring partner to acknowledge and honor the technique, when used in a non-destructive way. Maybe that is feasible, maybe it is not. It depends on the sparring partner, and the context and purpose of the sparring.
Grappling methods have a training advantage in this regard. Their methods can be used in a less-than-destructive way, and still be effective. Striking methods are at a training disadvantage in that regard. When they are done in a less-than-destructive way, they are often no longer effective. It becomes easy for the sparring partner to ignore the technique with which they were just hit.
Sparring can still be a useful exercise, but it depends on what someone is hoping to get from it. At the same time, it can undermine some important skills as well. It can train someone to habitually execute their methods in a non-destructive way, undermining the very purpose of the training. So people make an evaluation as to the relative worth. They may develop some skills at the detriment of others, and they may decide that is a worthwhile trade off, that the positives outweigh the negatives. Or they may feel that the trade off is not worth it, that the negatives outweigh the positives. That is a judgement that everyone needs to make for themselves.
But it is a gross oversimplification to say that people claim their stuff is "too deadly" for sparring. It is almost always an issue with much more nuance than that. Ive never actually heard someone make that claim.