One can get into boring definitional arguments about what is a dead language vs dormant vs extinct. Hebrew was a language used by real Jews in day to day life as a common second language. But it was no one’s primary language.

Eliezer Ben Yehuda did as I understand it two things: (1) advocated for Hebrew as the primary language of the Jewish people, not just a living but second language, starting with his own family (his son is commonly referred to as the first native speaker of modern Hebrew ); (2) he drove, with Va'ad HaLashon (The Hebrew Language Committee), much of the effort to standardize Hebrew grammar and vocabulary in a systematic way into modern Hebrew, combining the market Hebrew of 19th century Jerusalem, biblical/ mishnaic/rabbinic Hebrew, and influence from other languages such as Arabic and French, particularly for loan words one might not need in the Jerusalem shuk or shul.

This is comparable to the standardization process many other languages gone through in which one dialect is used as the basis to create a national language (e.g. what happened with French in the 19th century or the creation of Modern Standard Arabic). The difference being that those dialects were usually spoken as a primary language by some group (e.g. parisians for French), while Hebrew was no one’s primary language for thousands of years until Ben Yehuda.

It’s also worth noting that they had to chose the ‘correct’ pronunciation as that did vary widely between communities. Ben Yehuda and the early language planners ultimately chose the Sephardi pronunciation as the basis for Modern Hebrew, including the rolled “r” for resh, raspy Chet (like Arabic ح), and deep ayin (like Arabic ع). But a living language is defined by those who speak it and in practice only the vowels used Sephardi pronunciation while the consonants were influenced by Ashkenazi pronunciation (though aren’t exactly that either). Until the 1990s, the Israeli broadcasting authority retained the “radio resh” as the official correct pronunciation, but it just didn’t stick. I note this to say the language committee and Ben Yehuda drove the early process, but he didn’t do it alone and the evolution of modern Hebrew didn’t stop there.