To me, the words for SSI most concretely imply no PB. The user does not have proficiency with it. The only applicable modifier is as stated the creator's Int.
Note the similar language in counterspell, which many at first misinterpret.
SSI replicates a spell effect. Counterspell is an ability check. Definitely not the same thing.
The SSI doesn't make any reference to how the ability modifier is even applied if you remove the general formula so your argument seems a bit of a stretch. ;-)