I don’t see why some posters are taking offence at what is an objective question.
I am not seeing the responses as offence but perhaps frustration because you are not being anywhere near as objective as you think you are and you keep jumping to pattern confirmation when the numbers on this thread are much too small to draw a conclusion like that. What this kind of data in this kind of thread can give you is an idea of the range of experiences, but not the number/distribution. So if you see at least one response saying they have taught classes with 0-2 students with these diagnoses, that is an experience that exists (assuming we take all posts at face value and assume that nobody is lying or making up a completely random answer, which people do BTW). But it's unlikely to be accurate to be able to extrapolate well, only one person said that and everyone else said it's 20-40% so 20-40% must be more frequent. You can't accurately gauge that from a forum discussion thread.
Also - there are people responding just saying ADHD/Autism, some referring to all ND, some referring to all SEND. These are not the same thing, so it's confusing the numbers and making them not really comparable. The 1% in the OP is related to autism, I think? Whereas official SEND figures are about 20% so already extremely different to the 1% because that 1% is only one presentation of SEND needs, most of which are not related to autism.
There are some responding with diagnosed figures, some responding with suspected numbers. Some are responding with a whole school figure, an average, or their current class and some are responding with classes they have known of or had experience of in previous years. As some other posters have said, there's a psychological thing where when responses are open for everyone to read, it tends to put people off responding if their answer would be extremely different to the other responses (probably becuse it induces doubt about the validity of the answer) and I also think conversely, people feel more drawn to respond if they would be the first person to report a particular extreme, rather than repeating what others have already said, so someone with this experience might be more likely to respond. Because of the small nature of class sizes, you can't get any more extreme than 1/0 pupils in the downward direction, so people might be more inclined to post if the current max is 10 and they have experienced 13 (to borrow one example).