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?
Again, unless you provide some _other_ piece of data (full real name, email, etc) connecting that data back to you, I fail to see the harm. Regardless, no one is forcing you to use the site, or even provide real numbers. If anonymously providing your salary and savings data worries you then don't use it.
They're asking for even worse - all your historical financial data. Why would you want to give some random guy on the internet that?