"Those are not people."
Translation:To nejsou lidé.
It is similar as "It is John." "It is Jane.", you use it and not he or she. You can even say "It is them.".
It is this kind of "to"/"it". It has even more use cases in Czech.
However, in this particular sentence without any context given, I could also imagine a translation with "Tamti/Tamhleti" even with "Ti". That is even closer to the original "These". In this case the "ti" is not the plural of "to" (plural of to is ta).
Consider: "There are fellow citizens among us who did cruel acts. Those are not people." "Jsou mezi námi spoluobčané, kteří konali kruté skutky. Ti nejsou lidé.".