I provided an answer recently on examples of 'colonial genocide' in Africa which might be interesting, and from which you could possibly draw your own conclusions. The further reading in that answer also addresses many of the aspects of this topic. It is very difficult to look at colonial policies as being 'literally Hitler' through an objective lens because obviously the circumstances and specific conditions of each colonial example I provided in the answer above are very, very, very crucial to the actions and events that occured. I also think it is wrong to see colonialism as having been a top-down implementation - the policies of the metropole governments may have had little bearing on the actions of the troops/officials/administration on the ground, or the colonialist measures may have been direct interpretations of broader metropolic policies (Congo Free State, although again the nature of that private state is even more complex!).

However, I think it should also be noted that Césaire was writing in the 1950s when decolonization was in full swing and the full horrors of the Third Reich's actions were becoming known, both of which certainly affected not only the discourse within which Césaire converses but the narrative which he is trying to address - that Hitler's actions were bad, but not unique in the contemporary world, and that colonialism needed to be looked at more thoroughly within this revised worldview. Césaire's writing is heavily critical of both colonialism as a process and structure, but also the presentation of colonialism and I would be very tempted to point out that associating colonial actions with the mass killings of the Holocaust is quite simply a way to shock the reader. As Césaire states later on

the commonest curse is to be the dupe in good faith of a collective hypocrisy that cleverly misrepresents problems, the better to legitimize the hateful solutions provided for them. (p.172)

Césaire was writing with a very specific agenda which was to challenge the public perception of colonialism, and attacks it from a decidedly Marxist perspective of colonialism, and the idegology behind it, as having been an agent of proletarianization and mystification.

I understand this may not directly answer your question, but I hope this helps to understand why Césaire might be making this statement. There is a good little critique here that helps contextualise the arguments put forward by Césaire. http://www.rlwclarke.net/courses/LITS3303/2007-2008/06CCesaireDiscourseonColonialism(Belgrave&Hurdle).pdf

Edit: Spelling!

My response to a question in another thread points to one answer: there is a potentially more linear relationship between the genocidal practices of the Nazis and earlier German colonial behavior--and racial rhetoric--in what is now Namibia.

I would argue that Germany's behavior in Namibia was unusual in that Germany carried out a proto-genocide, hunting down people of a particular ethnic background, isolating them in camps, and exterminating them. Violence was an integral part of colonization and, of course, also of the slave trade; the systematized extermination of an entire people, though, is less common.

However, Césaire's larger argument is very important and ought to be considered from the perspective of whether or not he has a point, regardless of whether or not his version of history is perfectly accurate. By this I mean: how can we reconcile the white world's outrage at the horrors of the Third Reich with its relative indifference to the suffering and atrocities imposed so often by the colonial powers? Is there comparative indifference? Or is Césaire glossing over internal resistance within the metropole?

It's a question worth asking, even if we disagree with Césaire's conclusions (that is to say, even if we determine that race is not the most significant factor in how much outrage is generated by atrocities).