I'm trying to understand the concern but most of my worry around "historical financial data" would be actual transaction-level information, not rough summary figures to estimate a financial plan.
I just played around with the site myself and I didn't see anywhere that it imports data, plus the tagline on the homepage says "never ask to link your financial accounts"?
Isn't this essentially the same risk profile as like FIREcalc [1], or the NYT Rent or Buy calculator [2], or some portfolio analysis tool [3], are you saying that those shouldn't be online calculators either?
[2] https://www.nytimes.com/interactive/2014/upshot/buy-rent-cal...