Interestingly it's the other way around. Apple is using TSMC's 5nm process (they don't have their own fabs), which is better than Intel's in-house fabs, so it's Intel's vertical integration which is hurting them compared to the non-vertically integrated Apple.
Also, the answer to "is this only possible because of vertical integration" is always no. Intel and Microsoft regularly coordinate to make hardware and software work together. Intel is one of the largest contributors to the Linux kernel, even though they don't "own" it. Two companies coordinating with one another can do anything they could do as an individual company.
Sometimes the efficiency of this is lower because there are communication barriers and isn't a single chain of command. But sometimes it's higher because you don't have internal politics screwing everything up when the designers would be happy with outsourcing to TSMC because they have a competitive advantage, but the common CEO knows that would enrich a competitor and trash their internal investment in their own fabs, and forces the decision that leads to less competitive products.
During the iPod era, Toshiba's 1.8in HD production was exclusively Apple's only for music players, but Apple gets all the 5nm output from TSMC for a period of time.
Both teams deserve huge praise for the tight coordination and unreal execution.
Intel tried this with Itanium a while back and failed because it is difficult to get software developers to target a new isa and provide compilers and compiled code for everything unless you use a translation layer.
Apple is one step ahead here because their compilers already supported ARM isa (because iPhones use them) and had both the OS and apps ready to go from day one of availability.
They also had translation technology that would allow mutating x86_64 code to ARM64 code so that old apps would (on the whole) run acceptably fast on the new chip.
To do the latter properly, Apple had to create a special mode to run the arm chip with total store order for memory writes, which is not standard on arm. (It would be a lot slower if they didn’t have that when running Rosetta translated code.)
So both the OS being available, and the OS influencing the ARM tweaks (eg TSO) could they pull it off.
They also have the position that they build hardware that uses those chips so can mass produce - and in fact, replace - existing hardware.
Each of these things could be done in isolation by Intel/Windows/Apps but it would be difficult to do all three.
Even getting JavaScript maths in a special instruction was difficult enough on Intel, and that was something of benefit to any browser.
My guess is you’ll see Intel and AMD offering Arm chips in the near future, as both AWS (graviton) and Apple have shown the way to a new ARM future.
Without AMD everyone would eventually be dragged into adopting Itanium.
1. Fast uncontended atomics. Speeds up reference counting which is used heavily by Objective-C code base (and Swift). Increase is massive comparing to Intel.
2. Guaranteed instruction ordering mode. Allows for faster Arm code to be produced by Rosetta when emulating x86. Without it emulation overhead would be much bigger (similar to what Microsoft is experiencing).