You're obviously the man to ask, Jim: I haven't read Inspectre so is it definately absolutely a retcon and not character POV or potentially some sort of in-universe historical revisionism on the part of Sov propagandists?
I'm enjoying the old-school feel to Samizdat Squad, but find it unlikely that Arthur was unaware of the Inspectre or that Matt would want to actively retcon anything Dreddworld-related when it could be written around or included in canon with minimal effort. Recent Ewing tales have made a virtue of referencing continuity anachronisms like Judgment Day and Arthur himself has contributed at least one Black Museum tale that referenced the Death Belt not referred to since Apocalypse War (which predates the Inspectre by at least 10 years) so I'm inclined to think there might be an in-story rationale for any continuity 'revisionism' even if we haven't seen it yet. This is the House of Tharg, after all, not DC or Marvel.