Specifying font variant fails on macOS
font_family SF Mono Medium
italic_font SF Mono Medium Italic
bold_font SF Mono Semibold
bold_italic_font SF Mono Semibold Italic

Using auto works fine.
font_family SF Mono
italic_font auto
bold_font auto
bold_italic_font auto
Unfortuantely, I dont have an OS X machine capable of running kitty, only building it, so debugging font rendering issues is rather difficult for me. I have not really looked into font variant handling in OSX, but the relevant code is in core_text.py (set_font_family()) and core_text.m new()
Probably some some further attributes need to be set on the font_attributes dictionary.
The correct syntax for choosing SF Mono Semibold as your bold font is.
font_family SF Mono Medium
italic_font SF Mono Medium
bold_font SF Mono Semibold
bold_italic_font SF Mono Semibold
Most helpful comment
The correct syntax for choosing SF Mono Semibold as your bold font is.