A Chromium fork could replace certain components if they so choose. The PDF rendering component would be one of the easier ones to replace.

However, I was able to confirm that Edge uses extension ID mhjfbmdgcfjbbpaeojofohoefgiehjai to render PDF internally[1], same as Chrome, so indeed it's using PDFium.

[1] The rendered PDF element would look like

  <embed id="plugin" type="application/x-google-chrome-pdf" src="..." stream-url="chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/..." headers="..." ...>

And here's the extension's manifest in Chromium source, where you can find the extension ID: https://github.com/chromium/chromium/blob/2baa2b094cdd60e980...