The whole "clustering" thing is nonsense, but there is a real thing, known as "multimorbidity". There are conditions that are often found together, such that if you diagnose one, it's worth checking for the others. This isn't because the conditions are the same, but because some causal phenomenon (usually developmental) creates the circumstances for more than one condition. Premature birth appears somewhere in the causal network in a lot of cases, as does poverty. (I'm pretty sure poverty's a cause, though premature birth might be a result of the circumstances, rather than a cause.)

On that note, Wikipedia has a much better list than this PDF: https://en.wikipedia.org/wiki/Conditions_comorbid_to_autism. (This list is not without its flaws, but I at least find it easier to compensate for academic ignorance than magical thinking.)

Also related is differential diagnosis, which is a process by which clinicians distinguish between different diagnoses that have overlapping symptoms. (It's not a particular fancy technique, it's literally just the name for any approach that does that.)