The problem is that he said he does't want anyone from an almost entirely black country and instead wants anyone from the whitest nation on the planet, without any reference to how competent and qualified those individuals actually are. That is the definition of racism.
No, that is not the definition of racism. It could be about wealth and education level.
No, it couldn't about those things. If it was, then he would have simply said, "We want wealthy, educated immigrants.", without reference to country, let alone without using a 94% white country as a contrast to a 2% white country.
Countries do not immigrate, people do. So, things like nation level GDP and education are irrelevant. Both Norway and the various non-white "shitholes" Trump referred to have wide variance in how educated and wealthy people are, and it is mostly the less wealthy Norwegians wanting to immigrate to the US. Allow a random Norwegian rather than a random "shithole" resident to immigrate and you will sometimes get a less educated or less wealthy immigrant. The only thing you are almost certain to get with that policy is a white immigrant rather than a black one. Thus, the sole motive for such a policy would be to ensure the race of the immigrant.
Also, there are a number of countries with higher average wealth or education than Norway. But those superior countries have few whites, except for Luxembourg, which is still far more racially mixed and about 30% "swarthy" compared to Norway.
Conflating race with other things (economic status and education level are big ones) is an obsession of Democrats, as is labelling people as racist deplorables, so we really can't rely on clear thinking here.
Conflating nation-level averages with what is true of individuals is an obsession of conservatives and most "libertarians", because it allows them to mask their racist beliefs with bullshit about "education", "ability", or "income".
Yes, sometimes Dems take comments about individuals and turn them into strawmen assertions about whole groups, thereby making the speaker racist. This is not one of those times. This time the racist made a clear statement about entire groups that differ from each other far more by race than any other variable, and called one group "shitholes" who he didn't want in his country.
It is the definition of a smoking gun. Maybe its not analogous to directly seeing a person shoot another. But it is very equivalent to hearing a shot, running into the room and seeing a person standing over another's dead body with a smoking gun. Sure, there are far less plausible convoluted excuses that have a 1 in a million chance to be true, but any rational person would find that alone sufficient evidence to conclude he shot them. And in this case, with Trump countless other acts of racism, it is like the guy with the smoking gun has been found near countless murdered people before.