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It's not an unreasonable claim, but it depends on what you mean by equivalent.
Sephardi Jews are, ethnically speaking, the descendants of the Jews that lived in the Iberian Peninsula before they were expelled due to the Alhambra Decree of 1492. Many Sephardi Jews spread to North Africa and various European countries after their expulsion from the Iberian Peninsula. Due to this diaspora, Judaeo-Spanish (Ladino) is not the only historical language spoken by Sephardi Jews. Judaeo-Catalan, Judaeo-Portugeuse, Haketia (which is heavily influenced by Moroccan Arabic, and Shuadit (Judaeo-Occitan) were all spoken by Sephardi Jews.
Yiddish, conversely, is the primary historical language of the Ashkenazi Jews. Ashkenazi Jews spread far and wide throughout Europe and beyond, but seemed to keep Yiddish as a constant. Yiddish became a secular, cultural force in Eastern European Ashkenazi Jews, whereas German overtook Yiddish in Western Europe.
The Holocaust had a severe impact on the Ashkenazi Jewish population and the number of Yiddish speakers in the world. There are around 11 million Ashkenazi Jews in the world, primarily in the United States and Israel. Conversely, there are only about 2 million Sephardi Jews in the world, primarily in Israel, France, and the United States. There are just over 100,000 speakers of Ladino in the world, and most of the other historical languages of Sephardi Jews have died out. Yiddish has around 1.5 million speakers for comparison.
The majority of Sephardi and Ashkenazi Jews speak languages other than Ladino or Yiddish respectively, and most speak the local language of their nation. Ladino was far from the only historical language for Sephardi Jews, but was the language with the greatest longevity. Yiddish has had staying power for Ashkenazi Jews and remains a popular language for many Jews.
In short, Ladino is the largest surviving historical language of Sephardi Jews, but does not have nearly the number of speakers, preservation, or ongoing culture that Yiddish has.
Sources: Ethnologue: Yiddish