Author is just ignorant to the technicals and laser focusing on some particular cases that he thinks are problem but are not.

The redis engineers KNOW fork-to-save will at most result in few tens of MBs of extra memory used in vast majority of cases and benefit of seamless saving. Like, there is a theoretical where it uses double the memory but it would require all the data in database be replaced during short interval snapshot is saved and that's just unrealistic

Why? Most COWed pages will remain untouched. They only need to allocate when touched.