Note that the i-switch is not documented in the help file. As you note, it is used for various other draw commands for 'inverse'. However, for /drawtext, which uses a font, I would argue that -i ia somewhat expected to mean italics.
Normally, if a command is part of a series of commands, an undocumented switch usually means the same thing as what it meant for the previous command on the same help page. And that would be the /drawdot command that says -i is inverse. There is already a -p switch that lets you italicize (or use other control codes) in the text, so I think -i should be inverse.