Okay, so going by this logic Laws sword must've eaten the Ope Ope no Mi? We know this isn't true, and the same applies to Issho.
For an object to use its Devil Fruit powers it must have a conscience (as Issho can't use something elses powers remotely, hence you're implying the sword has its own consciousness), the only devil fruits with a conscience are Zoans due to their animal powers.
To support this; the only objects to eat a Devil Fruit are Zoan users
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Now, about the issues of confusing, concerning his sword usage. Doesn't law use his sword with everything?
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Issho is no different. He simply chooses to use his sword as a platform with which to direct his gravitational forces.