There is very likely a difference between some given email address appearing in a list of accounts from a specific breach and some given email address "appearing on the dark web". The "dark web" is akin to "the seedy side of town" and has similarly vague identifiable boundaries (Making is equally vague as to what "scanning" it means exactly: There isn't a search engine for these Onion addresses, and they tend to come and go rapidly). I just ran the scan at both and Google's results had a throwaway address of mine listed as part of some aggregate lists and general appearances unaffiliated with a specific breach. haveibeenpwned, on the other hand, listed each breach and its date.I don't think that does a very good job of scanning for a breach. If I run that I get zero hits. But if I run the Google tool to scan the dark web I get two hits.
Probably smart to use both.