yes, i've tried on KT 5.1.0 and it would not synthetically embolden regular fonts. i've even tried the following config, but was also unsuccessful.
Synthetic emboldening for fonts that do not have bold face available
<!-- check to see if the font is just regular -->
<test name="weight" compare="less_eq">
<!-- check to see if the pattern requests bold -->
<test target="pattern" name="weight" compare="more">
set the embolden flag
needed for applications using cairo, e.g. gucharmap, gedit, ...
<edit name="embolden" mode="assign">
set weight to bold
needed for applications using Xft directly, e.g. Firefox, ...
<edit name="weight" mode="assign">
not sure if it will help if you install the freetype library available on this forum.