Well, I traced the call and realized it can't be my php4 since it's definitely calling html2text.php over the web which is hitting php5 -- but for some reason the autoload just doesn't work.
Rather than pull my hair out trying to figure that out, I just bypassed autoload and required the appropriate file directly.
in bin/html2text.php
at line 6 (Before the line $converter = ...) add this to get it working:
// autoload not working? had to add this line.
require INSTALL_PATH.'program/lib/html2text.php';