Actually the presidential candidate was selected on the day of the convention up until around 1970. They had conventions that would last days, they would lock the doors until a candidate was picked. Then ideally everyone stacked behind the final candidate.
I’m pretty sure there was actually almost a contested convention in like 1980. So no, this is objectively wrong and a bit of a modern convention.
Doing more research, candidates that were picked from contested conventions are absolutely less likely to win in historically.
https://www.pewresearch.org/short-reads/2016/02/04/contested-presidential-conventions-and-why-parties-try-to-avoid-them/
So close enough with a candidate as strong with his party as Trump.