The goalpost remains exactly where it always has. How does evidence of Einsatzgruppen activity prove the gas chambers?
Well, for starters the Einsatzgruppen operated gas vans, which is documented, and they even left mass graves with victims of the vans intact so that Soviet pathologists could observe unbloodied corpses of dead civilians next to locations where other sources indicated gas vans had been used.
Either Nazi Germany gassed people or it didn't. If it did, then further claims of gassing are more plausible. Indeed, the fact that there are so many cases of the use of gassing to kill people in the Third Reich poses a major problem for deniers, one that is not resolved by your Treblinka fixation.
The evolution of gassing as a killing technique came directly out of the euthanasia program, which is documented fairly well, and also encompassed early experiments with vans, then a second generation improved by one branch of the SS, the RSHA. Meanwhile, the WVHA improvised then developed gas chambers of its own, after encountering the euthanasia program through action 14 f 13, and then experimented on Soviet POWs, following on from the commissar order, in at least two camps, including Auschwitz. Thereafter, the SS at Auschwitz improved on the idea and built new crematoria with gas chambers, as is documented. A third branch were the regional commands or SSPFs and HSSPFs, who cooperated closely with the other major branches. Globocnik, SSPF Lublin, conducted some experiments with gassing, again independently of the other ventures, and then received specialists from the euthanasia program. Koppe, HSSPF Warthegau, knew some guys who had been running around killing psychiatric patients in vans, and then in conjunction with the local party boss Greiser as well as his boss Himmler, set up Chelmno, another camp which is documented.
That is a thumbnail sketch of the evolution of gassing, as it has stood ever since the late 1940s or so. It places gassing in the context of the Final Solution directly in connection with the euthanasia program and the Einsatzgruppen. So the evidence is of a piece, which is why histories of the Holocaust tend to discuss all these things together.
Since the Final Solution wasn't only carried out by gassing but also by the use of copious amounts of bullets, not just by the Einsatzgruppen but by the Police, Waffen-SS and other formations, then if we're discussing the Final Solution, aka Nazi extermination policy, aka the Holocaust (albeit not all of the Holocaust, but undoubtedly what made it become 'the Holocaust'), quite clearly the Einsatzgruppen etc are part of the Holocaust. In some regions gassing was used and then shooting became the major method, so that the two simply cannot be separated, eg in Galicia, which is 10% of the Holocaust as a whole.
It doesn't really matter which way you slice it, the extermination camps are at the centre of a Venn diagram, with multiple overlapping circles.
This is important when considering typical denier handwaves like 'all Jews are liars' since we clearly have Jewish witnesses to shooting and to gassing. Also when considering other denier handwaves like 'Nazi policy was only emigration/resettlement' since if the Nazis wiped out virtually all Jews in the occupied Soviet Union using largely bullets, then the handwave is refuted, and it makes it hard for deniers to argue that the deportations to the camps were actually destined to go to the 'Russian east' in some idyllic reservation plan, when the native Jews were being slaughtered.
The evidence is connected because it belongs in the same narrative and same explanation. That is just how the Holocaust is conventionally understood. Therefore 'gas chamber skeptics' are inevitably going to be asked how their skepticism copes with the rest of the Holocaust, since there are major logical, narrative and evidentiary problems in the apparent stance of doubting and denying the extermination camps alone.
