They don't believe anything has changed in the MP since Sergius - that the MP hasn't been repentant of the Sergianist heresy (which isn't entirely false... that giant church they are building as a memorial for the "Great Patriotic War.")
Theologically, their justification for remaining in conflict with the MP looks like a moot point. Their argument that the MP was undermined by having relations with the Soviet government would be logical in 1920-1990, but once the Soviet government fell and was replaced with a Russian government without the Atheist leanings/tendencies/policies of the Soviet government, the point becomes moot.
The MP's policy of reconciliation or compromise with the Soviets, to the extent that it was ecclesiastically undue, was not really something that the MP preferred, but rather forced on it by circumstances. To object to the MP still over this and still hold a grudge against the MP 30 years later sounds like Donatism. Sergianism today, ie reconciling with the Soviets, is not really a "doctrine" that the MP needs to repent of. It's a policy that is now a moot point theologically.
To give an analogy, if some Greeks broke off from the EP because the EP was under Turkey, it would make sense if they wanted to be out of communion with the EP or wanted to set up an "EP IN EXILE" group. But if Istanbul was retaken by the Greeks, it would become a moot point. There would be no practical , need for the EP to repent of its past with the Turks, because we are talking about a policy from 30 years earlier, not an ongoing theological conflict.
I can see a practical concern if they are worried that rejoining the MP could lead to the Kremlin potentially influencing them, but that practical concern also becomes moot when you consider the OCA as an option for them.
I can see that they are upset about past history regarding the OCA and MP regarding relations with the USSR's government, but like I said, we are talking about something from 30 years ago regarding a government that is not Atheist-promoting since then, and is arguably Orthodox today. Christians are supposed to aim for reconciliation and unity and put political histories like that aside.