I doubt they provide any kind of guarantee that they'll catch everything. That's not what a tool like this is used for (i.e. it's not a security tool or a network monitoring service).

OSINT is about gathering data from public or semi-public places.

There are plenty of private dark web forums that these kinds of tools probably don't have visibility into, but the bigger public well-known ones are where you are most likely to see high-profile breaches being sold (since there's a wider audience and actors want to sell data quickly before it devalues), so it's certainly better than nothing...