After a few tests it looks good but it reports some domains as registered when they are not (for instance kostikaable.com or kostikaition.io are marked registered but do not seem to exist when I manually run a Whois). Maybe the tool is running onto some rate limiting thresholds?

Also when there are a lot of domains to check you could check them in parallel, otherwise it gets quite slow to wait for the results (but that might be an inherent limitation of the Whois servers).