Poor grammar was used in this sentence. I found it was simply a comment someone made on a Facebook post. "I find this so amazing! That even on a massive clothing brand like Boohoo they haven't photoshopped away the models stretch marks!" If she had written it properly, it would have looked like this: "I find it so amazing that even a massive clothing brand like Boohoo hasn't photoshopped away the model's stretch marks!"

As it is it doesn’t work properly as a sentence, it looks like a sentence fragment. There needs to be something for the ‘that’ to refer to.

It sounds like that bit of writing hasn't been taken from a full sentence. It would make more sense for it to start the way you mentioned. I sounds like the full sentence may have started something like. It's wierd that even Boohoo etc...