The concept of "admin" is not a black and white concept, it's a spectrum.
Every iPad user is an "admin" of their device. Apple isn't stopping you from "administering" it.
There are limitations to the device but every device has limitations somewhere.
On that spectrum, the iPad is a much less limited device from a software and hardware standpoint than a lot of other devices, such as game consoles, point of sale terminals, medical devices, etc.
A lot of devices are intentionally designed with safety guardrails for very good reason.
For example, an inexperienced end user should not be able to freely reprogram the firmware on a robotics system used for surgery. A system like that is very intentionally shipped with guardrails that prevent the owner of the device from making dangerous modifications.
Other guardrails exist for other hardware where the end user is given some freedom but still kept from destroying the hardware or harming themselves. For example, your processor or graphics card won't let you fry the system by exceeding thermal limits, but they may still allow you to change settings like clock speeds and voltages. These limitations generally make sense and I would not really appreciate if AMD allowed me to accidentally fry my processor. A car infotainment system that allowed you to play videos or games while you drive or disable driver attention monitoring during self-driving would be grossly negligent.
I personally think the "everything should and must be open source" crowd takes a good concept way too far and essentially wants to dictate business models and approaches to safety and security.
I would support a more moderate approach that would force Apple to support more of a level playing field for things like alternative app stores, interoperability, and fair competition with alternative software, software/hardware end of support anti-e-waste regulations, but forcing them to divulge schematics and allow you to modify sensitive parts of the hardware like the secure enclave is not particularly beneficial. Just my opinion, though.
And finally, I’m firmly in the camp that I’m not going to tolerate a shitty solution just because it’s open source. No, I’m not going to use a $1000 Librem 11 tablet that’s at least 4x-12x slower than an iPad Pro that costs the same amount of money.