To me racism has a pretty simple litmus test - would you treat an individual of another race, but in an otherwise identical background situation differently? If yes, then it's probably racist. If not, then it's probably not. So Tuskegee has some interesting backstory. It was inspired by a similar experiment that was carried out in Norway that followed the progression of untreated syphilis in thousands of people over decades. [1]

That study provided extensive data and information on the progression of syphilis, but at the time it was believed that syphilis affected different races in different ways. And black Americans had (and still have) infection rates dramatically higher than other major groups. So this meant that studying this exact group could not only be overall most impactful on a population basis, but was also the least well understood group (as the Norwegian study presumably lacked much of anybody of African ancestry) and so the most most likely to yield novel/informative science.

So if these individuals had otherwise been just another subgroup of whites (but one still had reason to think syphilis might affect them differently than e.g. Norwegians), would we still have carried out this experiment? It's impossible to say for certain, but I think the answer is probably yes. Not only was such experimentation already happening across the globe, including in relatively homogeneous societies, but there have been all sorts of other US government driven experiments on the population where white groups were just as readily experimented on like MKUltra [2], Operation Sea Spray [3], and so on endlessly.

[1] - https://www.sciencedirect.com/science/article/abs/pii/002196... (note the date on the paper)

[2] - https://en.wikipedia.org/wiki/MKUltra

[3] - https://en.wikipedia.org/wiki/Operation_Sea-Spray