I think you have to see it as a good thing, though it must be hard to hear. If she has put things like that, a mainstream school might still be appropriate if they adapt the classroom and hire someone with TEACCH experience and pay for the class teacher to have training in it too. This is the sort of thing that can be written in to the Statement.
Saying "X will require a higher level support that is usually available in a mainstream school" is nothing at all to worry about. It is a stock phrase to say that a Statement is necessary as he needs a higher level of resourcing/ funding than the school would be able to provide on its own (and that's the criteria for a Statement, so she's saying he needs one!)
These professionals have to walk a bit of a tightrope. They're discouraged from telling the full truth, but also need to provide a clear picture, so they have to kind of use words to hide it.
J was seen by the EP at age 3 and was totally totally out of control. Everyone who ever met him said he needed full time support at school. The EP had to put something woolly like 'will need a high level of support' and, like you, 'will need more support than would usually be available in mainstream school'. That was good because it got him a Statement.
You could have ended up with something totally woolly like 'will need some additional support' or, even worse, 'would benefit from extra support' - what she's written is really good because the LEA will listen to her and so she's sent a clear message that assistance is needed. They have to spell it out a bit though and sounds like she got as close to that as she would be allowed to!
If all these things go into the Statement, the school HAVE TO do them. So it's good that she put them.
It doesn't mean he's not doing well. It means you got a good SALT who's willing to be as honest as she's allowed to to get the resources for you that DS needs.