Samuel Beckett- Question on Writing in French and Translating to English

I just finished reading Molloy by Samuel Beckett. It was the first novel of Beckett's I've read, and it was incredible. I have always heard that Beckett wrote many of his works in French (including Molloy ) and then later translated them into his native English. That fact alone simply blows me away. But in reading Molloy , I was struck by the vocabulary Beckett employs, as I was reaching for the dictionary every page or two. My question is this, for anybody familiar with Beckett in the French and the English- did he use equivalent terms/vocabulary in French? I spot checked two words that were new to me, fulgurate and absterge, and they both apparently have direct counterparts, cognates to boot, in French. Did he use those words, and all the other equivalents, in the original French? I'm just trying to wrap my head around just what his command of French was.