The SLV calls do not belong on 6781 but "Even if you did do an options straddle you would not report it on Form 6781" is false.
This form is used for 1256 contracts (futures) but also for any identified section 1092 straddles. Not just straddles on futures. A regular options "straddle" is covered under section 1092 and reported on this form.
https://irc.bloombergtax.com/public/uscode/doc/irc/section_1092
This is also addressed in Pub 590, Chapter 4
