Perhaps it would be helpful in conveying your message if you provided examples that are not entirely abstract. Try to give an example of a real choice in CIV IV and then how that choice is handled in CIV VI.
Sure... too easy. From the top of my head without even thinking too much:
1. Science infrastructure: in 4, you have to consider where you want to build up your science city, where to put your Academy (ies), libraries, etc... city specialization matters. In 6, you can spam SDs and their buildings, it does not matter where... yeah yeah, adjacency bonus yadayadyada, but in the end even that does not matter, you still catch up and surpass the AIs. Fake choice.
2. Civics, Policies, Cards: in 4, you have to carefully consider which civic to run according to the game situation at any time, the diplo relations, your plan, your VC, etcetcetc... in 6, you run cards. Any. It does not affect the end result. It affects the short term somewhat, but the end result is the same. You win. Fake, but cool, choices.
3. Gold: in 4, the economy is a struggle, especially in the beginning, but also during the entire game. The "dreaded" sliders (Oh the horror) make you consider your investment mix at all times, and you have to consider where to get the commerce from in the first place, and at all times. In 6, you spam CDs, and swim in gold. It does not matter where you put the CDs. Fake choice.
4. Diplomacy: in 4, there is no "befriend everyone", too many factors come into play making pleasing everyone an utopia (real life anyone?). You have to consider multiple factors at any time depending on the game situation: religion, vicinity, trading, leader personality, resources, technologies, etcetcetc. In 6, you can be everyone's ally, or make everyone hate you, or anything in between, it does not affect the end result. Fake choice.
5. Golden Age: in 4, getting a GA is something influential, and timing it even more so. It affects the gameplay, and if timed correctly, the end result. In 6, you get a bucket to fill with the obvious. Then you get a less-golden age, or a golden age, or a little-more-golden age, which mean nothing in the gameplay (well, some loyalty here and there, but easy to compensate). GAs mean nothing in the end result. Fake pseudo-choice.
And on and on. What do you want me to do? To write a PhD thesis explaining the obvious to some? Maybe they should just go and test both games and compare.