This dataset was taken from scraping the company pages from LinkedIn. A company has to pay to have this page, so this certainly does not include all companies. If you have a premium account your search is not rate limited so you can iteratively scrape anything you want even though it's technically a violation of the terms of service.
There are many companies that sell data scraped from LinkedIn as a product. LinkedIn won a court case against hiQ Labs for scraping member data and other things[1]. I am not trying to compare this court case to the OP's website, just something worth mentioning.
In any case, web scraping is a sort of gray area of the law. In my opinion, this data set does not contain member data and is not being monetized so it feels kosher to me.
(Opinions expressed are solely my own and do not express the views or opinions of my employer.)
[1] https://www2.staffingindustry.com/Editorial/IT-Staffing-Repo...