Some anonymous guy on Internet says so.

Their source/verification of validity is "trust me bro".

That said, the people publishing this do have actual journalist credentials: https://www.kenklippenstein.com/about

So it's not unthinkable they could indeed get a copy, and if they did, based on their stated values they would publish it.

Major news outlets have it in their possession, have published a variety of select quotes that are found in this posting, have described the length of the manifesto as matching this posting, and have not denied the this published version was the manifesto found on the suspect. If that's not enough, then give it a day for more information to come out.

fake