I agree it's best to be skeptical here but folks are pinning this alleged planet nine at an apparent magnitude greater than 20/21 That's very faint, even by modern standards. While that's not out of reach in big telescopes, it's time intensive and you'd need to know bang on where to look.

That magnitude is near the limit of most of the ongoing all sky surveys, so it's not particularly surprising that it wouldn't yet show up that way were it to exist. In that respect at least I don't think it's fair to totally rule it out because we don't see it yet. That's way harder than it might initially might seem. Things as far away from the sun as this is proposed to be are going to be really difficult to detect via the ole' photon collection method regardless of size.