Vi was designed to work over a 300 baud modem. The throughput needed for a text editor is minimal (and still is). A C API would avoid the extra overhead, but no one is interested in writing cross platform unicode/binary safe plugins in the language. Also consider that buggy plugins that live in other processes won't crash the editor.
Although you're technically correct (since baud connotes the rate of change of the signal and not the rate of data communicated over that signal), modems didn't start sending more than one bit per baud until we got to higher data rates. At the time vi was written, 300 baud was in fact equivalent to 300 bps and the terms were used interchangeably.
