Interconnection is focussed on how an approval queue works, we're big fans of theres but this is about cost to build and how that changes over time.

iiuc interconnection.fyi has information available from interconnection requests which broadly speaking includes size, timeline, capacity factor etc. Whereas the halcyon product has information mined from regulatory filings. Regulatory filings include more contextual information like how much a utility paid for (or is planning on paying for) a particular plant/unit, as well as testimony by expert witnesses which includes other goodies like "X-company paid Y for Z-style of plant".

disclosure: I work at halcyon :)