I've always interpreted the notion of "autistics having limited imagination" to apply mostly to imagining what an other person/persons might be feeling/doing in a particular situation, and not so much imagination in the sense of dreaming up fantasy worlds, writing stories, or playing with toys. I think the psychiatrists who've throughout the years made the assessment of autistic children having 'limited imagination', have contrasted these children against non-autistic control groups, and have found that on average autistic children have a less sharp, while not non-existent, ability to put themselves into someone else's shoes. I think that's what's meant with an impairment in imagination in this particular case.
Also, the tendency of autistics to stick by particular rituals and be rigid in this aspect, as well as usually focusing on a particular set of interests (or one interest), might give off the impression to the outside world that we are unable (and not unwilling, which is more often the case), to look beyond our own little world or even 'think outside the box'.
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