I thought simple explanation for RYBZ is that he is not sure whether he was right in not asking Vespasian to save whole Yerushalaim. Continue the analogy, Alter Rebbe was absolutely confident in his opposition of Napoleon and support of the Russian czar.
Unlikely alternative history that Alter Rebbe was scared of: French take over Poland and Russia, Jews learn of progressive ideas, and many assimilate. Better slavery in Russia than that. At the end, Pilish/Russian Jews experiencedboth slavery and assimilation, so I am not sure why Alter Rebbe was so sure (and most of other misnagdim and chassidim disagreed)