It is one thing to cite (false) accounts of ethnic Russians being abused as a pretext for intervening in a neighbor's sovereign territory. It is another to cite the historical borders of a country that was twice as large not that long ago.
Know who else was Russian "in czarist times?" How about NATO members Latvia, Lithuania and Estonia, not to mention much of Poland, as well as non-aligned Finland. Know who else in Europe had larger borders before 1918 (i.e., in czarist times) than they do now? Hungary, Serbia, Germany, Turkey, Austria ... you get the picture. For example, I don't trust Hungary's semi-authoritarian, sharply nationalist government to resist the opportunity to restore its Habsburg-era borders if it got the chance, something that would tear NATO apart.
But let's keep the focus on Russia. Putin's citing of historical borders created by czarist conquest (which, of course, altered even older, more "historical" borders) opens up another justification for him to wreak havoc all over the area of the former Soviet Union.
As a historian, I can tell you that virtually all of today's borders (including our own) resulted in part from conquest and often resettlement. War has long been how disputes over territory are settled. But since the establishment of the United Nations, countries are not supposed to be allowed to take land from one another. More specifically, Russia agreed to respect Ukraine's borders when Kiev gave up its nuclear weapons 20 years ago. Putin has shredded that historical document.
And since we are on the subject of history, please remember that Putin once described the breakup of the USSR as the "greatest geopolitical catastrophe" of the previous century. I guess he likes some parts of history better than others. That's what concerns me about the future.