David Starkey was racist not by saying slavery is not genocide but by explaining the reason it wasn’t genocide was and you know what he said.
Anyway, in the case of slavery, the objective is not to kill the humans taken captive but to keep them alive for economic exploitation. In the case of the transatlantic slave trade, yes some did not survive the ocean voyage but I did read a history book regarding transports of convicts to the Americas and Australia and that they suffered similar mortality rates on the sea voyage over. There was no intent to kill people by shipping them across oceans, there was a mindset that the weak were going to die and that every voyage always had deaths on it from disease.
The Holocaust started as slavery. The Jews were initially forcibly placed in work camps and ghettos for exploitation and subjugation. The work camps were public work and agrarian, the ghettos they’d march groups to work without pay in military materiel and munitions factories. It wasn’t until the final solution days that the intent shifted that the work camps were converted to death camps, and the ghettos cleared out, transporting the remaining population to the death camps.
The same with the Japanese use of European prisoners in Indonesia. Started out as work camps, but then some particular areas crossed the line into genocide. Take for example the Burmese camps that built the railway of death. The prisoners were deliberately worked with not enough food to eat until they died. They were given no shelter, no clothing, no medical attention other than what they could make for themselves out of jungle plants.
So, it is important to recognise that slavery is often a gateway or enabler to later incidents of genocide. And I do think a large part of the trans Atlantic slave trade did in fact lead to genocide as while some destination plantations ranged from humane to brutal work camps, many many other plantations were most definitely death camps. The fact that many S American ports had to import millions more slaves than the N American ports due to the death rate of the slaves being over 90% is clear evidence that genocide was happening in many areas. Too often, we visualise the trans atlantic slave trade with the narrow focus on a Virginia tobacco or cotton plantation when most slaves did not experience that ending to their sea voyage. Many ended up worked to death in the jungles of S America.