Ask Poincaré. The Lebesgue integral was so important, so relevant to his eyes that he never took the trouble to say anything about it between 1904 (at the latest) and 1912 nor to discuss it with Lebesgue or Borel, as far as we know. See [An historical mystery : Poincaré’s silence on Lebesgue integral and measure theory?][1]

[1]: https://mathoverflow.net/questions/345275/an-historical-mystery-poincar%c3%a9-s-silence-on-lebesgue-integral-and-measure-theo#345284