About 50 people have already labelled this thread hilarious/awful, but I figured I'd weigh in anyway. I've listened to both bands since slightly before LFDGD and In Absentia came out, and have never once even thought of them as similar. I don't hear any PT in Katatonia and especially not vice-versa.
The only way I could see anyone making this comparison at all is if it's someone that's new to the "metal" subgenre. Typically the first bands someone is going to hear about when they make the move from Metallica or Tool or Dream Theater or whatever to slightly more "underground" stuff in the direction of Katatonia are going to include groups like Opeth, Porcupine Tree, In Flames, Katatonia and Meshuggah. And when your musical background consists of Metallica and those bands, you're obviously going to think Katatonia and Porcupine Tree sound somewhat similar, since they have some very loose similarities in their sound and are hard to compare to anything mainstream.
I know when I first started making steps away from Metallica, Dream Theater, and Megadeth back in about 1999 the bands I saw mentioned over and over were Amorphis, Anathema, The Gathering, and Opeth. Obviously at this point in time I don't see much similarity between most of those bands' sound, but at the time it all sounded like "that european metal" to me. Same thing prolly happened with this Navras guy and PT/Katatonia.