[wainguo]

It is easy to verify that:

One way to demonstrate that all processing is being done locally is to load the website, disconnect from the internet, and then use the site.

[holbue]

This is no proof that it doesn't send any data when used for conversions online.

It even could work offline, cache/store data and send it when back online.

[bl4kers]

No, it is not easy to verify without looking at the source code.