You need to find out what they mean.
My ds is bright, even very bright, particularly at maths, but I could have some sympathy with teachers describing him as unteachable. They never did, but reception had several times where he refused point blank to do something, or if he did it, it was so bad that it wasn't really worth it.
I remember the self portait that had 6 eyes and 2 mouths and painted green. 29 pink smiley faces and this... but he'd not wanted to do it, and that was one of his better objections. But if he really really didn't want to do it, he was quite happy to refuse to do anything for a whole day rather than write his name once, or whatever he was meant to do. He was stubborner than they were.
Any attempt to correct his writing, spelling, pencil grip, get him to write at times resulted in him moving his hand down to the very top end of the pencil and writing with it held in full fist. That sort of thing made it very difficult.
After a bit he did get bored of this and worked out the better way was to do it as the teacher wanted as quickly as possible, then he could get back to playing, but while it lasted it was hard for the teachers, and hard for all around-it basically required someone to sit with him and keep his focus of doing it, praising and chivvying him constantly.
I could do that 1-2-1 at home, but it isn't suitable for a school setting.
For him, yes they said "oh he's young for his age" etc. but although he's improved, in year 3, I still look at him and wonder if there's asomething not quite right. He's doing very well now. But, lets say he wasn't.
Then I would be kicking myself for not having looked and pushed for more intervention in year R.
Schools, ime don't suggest IEPs etc easily, so I would go with it. At worst he'll get some extra help he doesn't totally need-but would probably help him, as it would any other child, for a time until they decide he doesn't need it any more. But it may also stop the awful feeling of gradual realisation that he does need more help and it would have been better if it had stared sooner.