There might be something to that, in a way. Even if they can argue perjury happened in some way, it won't really make the intended splash, true or not. Since everything is viewed through a partisan lens these days, it would get lost in the mire of partisan bickering and back-and-forth in the minds of the public. So maybe they felt it just wasn't worthy enough to pursue. Sexual abuse allegations though, that's going to be taken seriously. That's going to make a splash. Yet even that is showing to be interpreted along partisan lines, even though we don't have enough information yet.
I honestly haven't looked too deeply into the perjury claims; I usually wait until something becomes a serious issue before I look into these things. But part of me also feels like Democratic opponents of Kavanaugh in the Senate wouldn't hold back if they had anything they could throw at him, so I dunno. Maybe they're keeping it in their back pocket if this allegation by Ford falls through. This is politics, after all.