Technically it is a 24 hour fast but - as you rightly point out - that is really a meaningless description.
For example, having a regular breakfast that will take until about 5 PM to really clear your digestive system followed by a 5 PM intake of a few ounces of raw spinach and then not eating for 24 hours is also a 24 hour fast.
But, the differences in how long insulin would be low (the point of fasting) would be huge.
The way I like to think of fasting is that it starts when you actually skip a meal you would have eaten (vs. the last moment you had food).
So, if you wake up on Monday and skip your normal breakfast - you are about 4 hours fasted at lunch time (which probably corresponds to about 4 hours of low insulin). Tacking on the previous 12 hours that included digesting and taking in the nutrients from your previous night’s dinner so you can call it a 16 hour fast doesn’t make sense to me.