O'Reilly customer support mentioned that the app is simply using system's default font.
There is this section in etc/fonts.xml, which is the same for Onyx Boox Note and Pixel phone
Code:
<family name="serif">
<font weight="400" style="normal">NotoSerif-Regular.ttf</font>
<font weight="700" style="normal">NotoSerif-Bold.ttf</font>
<font weight="400" style="italic">NotoSerif-Italic.ttf</font>
<font weight="700" style="italic">NotoSerif-BoldItalic.ttf</font>
</family>
Onyx Boox Note might indeed be using the NotoSerif-Bold font (which was also recognized by WhatTheFont as "Droid Serif Pro WGL Bold").
I don't know why O'Reilly app chose to use the bold version of the font on the Onyx Boox Note, and regular version on the Pixel phone. I'm wondering if it has something to do with the weight.
I tried to modify fonts.xml but wasn't able to, possibly because the device isn't rooted.