Because Gorby was all about 'admitting Soviet mistakes' which mostly meant accepting Western narratives (which were not accepted by mainstream Soviet Russian historians, and were incredibly controversial) with the idea of 'bridging the gap' between East and West. Like when you read Gorbachev, you get the idea that he was a liberal western-style communist, who saw inefficient parts of a system that did have aspects of Russian chauvinism and said, 'Well we can do better, look at those Nordic social democracies, let's transition to be more like them.' And then proceeded to unintentionally set the stage for the entire thing to get blown up by the vastly empowered criminal class.

Also, his entire legitimacy kinda rested on being reactionarily anti-Stalin.

Gorbachev “admitted” to it but this admission wasn’t based on archival records. Rather the evidence the admission was based on was of an “indirect” nature.

Gorbachev might have believed it to be true or maybe it was a political decision to demonstrate a clear break with the former USSR and a politically opportune gesture of goodwill to a neighboring country that was going through a nationalist moment of anti-soviet sentiment.

Whatever his motivation, he wasn’t speaking from personal knowledge or even archival records but from “indirect” evidence that he professed to believe.

That is true but doesn’t definitively answer who Goebbels thought was responsible. A diary entry from a few weeks later on May 8, 1943 states:

Unfortunately German munitions were found in the graves of Katyn. The question of how they got there needs clarification. It is either a case of munitions sold by us during the period of our friendly arrangement with the Soviet Russians, or of the Soviets themselves throwing these munitions into the graves. In any case it is essential that this incident be kept top secret. If it were to come to the knowledge of the enemy the whole Katyn affair would have to be dropped.

Who knows how reliable Goebbels’ narrative here is. But either way, he is not out-and-out admitting Germany did it. He implies the Soviets did it, but with German equipment.

Poles are obsessed with Russia, every Pole that has ever died must have been the victim of a secret Russian plot

I personally haven't come across a specific reason to doubt Grover Furr's truthfulness, but I have seen people express pretty strong opinions on it (but then again, that's exactly what the anticommunist orthodoxy would demand). Of his works I've only read Blood Lies, but it's kind of a special case because it's debunking a specific book from the book's own sources. So, even if you completely ignore every point Furr elaborates on with 'outside' sources, it still tears the communism=fascism narrative to shreds, and makes the point while doing so that Bloodlands is basically the pinnacle of anticommunist historiography.