Thanks guys for explaining that.
I went and studied what data/records get written to the different tables when it is done manually and I recreated what takes place.
The following adds a part with same name as the type - as kevink
def => def
field => field.OfType("TaxonomyField")
So now if you were to compare the database tables (mainly the [settings_*] tables) between the manual way and code way of adding the TaxonomyField to a ContentType - they are identical.
I was a bit disappointed to find out that the Field was still not showing up on the ContentType.
Do I need to allow something (module?) or perform a refresh/restart for the UI to reuptake the table modifications?
That alter takes place outside of the datamigrations class so that the taxonomy ID can be inserted into the setting.