It’s not a debate, seems more like a fringe theory trying to muddy the waters by making Russian imperialism look like colonialism, because the latter probably sounds worse to the modern average citizen.

If you look at e.g. https://en.m.wikipedia.org/wiki/Colonialism there’s a really long list of countries under British colonialism. Russia has such a short list, it’s no wonder it’s not considered a (former) colonial power.