In fact "bold" in your post appears bold. It's just not that heavy a stroke.
Look carefully, "bold" is in fact bold. At least on my Android table, in Chrome.
1. This is bold this is not.
2. This is bold this is not.
Using the default font.
#1 was achieved by typing "This is ", tapping B on the toolbar, typing "bold", tapping B on the toolbar, typing " is not.'.
#2 was achieved by typing the entire sentence then highlighting"bold" an tapping B on the toolbar.