Tx. You can use icu.sportSettings.ftp (or indoor_ftp). Note that the icu_ftp on the activity is copied from the sport settings when it is first analysed (choosing ftp or indoor_ftp) so history of FTP is preserved. If you use FTP from sport settings directly you are getting the current setting, not what it was when the activity was done.
You can find docs for what you can get from the icu object for an custom activity, interval or stream here: Server side data model for scripts