They were a cautious bunch, V.M. Slipher especially. Years of association with Percival Lowell’s brand of lunacy can leave you a little gun-shy.
But the “object” they’ve found is “one apparently fulfilling Lowell’s theoretical findings”. In other words, this strange, tiny body is just where it’s supposed to be — going by Percival Lowell’s decades of residual calculations. And it’s definitely “trans-Neptunian” — that is, it’s out past Neptune, and it’s retrograding. And the only thing that retrogrades is something in orbit around the sun.
So what is it, if not a planet?