I'd be interested in how true that is. All we really know is that there are strong morphological similarities between modern and ancient horseshoe crabs, to the extent that can be evidenced by the fossil record.

But there are many contemporaneous species that seem similar in form but have very stark differences in e.g. behavior.

Unfortunately DNA AFAIK degrades beyond recovery on geologically short timescales, so we might not ever be able to say anything definitive.