How can iPad battery remain on 100% charge over night?
I assume most (if not all) the other devices you refer to were various Android models? If so, how long has it been since you've used a device with a very recent version of Android?

The reason I ask is that Apple's iOS has always been more efficient in its battery management. This is why Apple gets away with using batteries of lower mAh ratings. And it's the reason why Google launched Project Volta - because they needed to make Android more efficient at its core.

In a nutshell, Project Volta resulted in improved efficiency starting with Android 5.0 Lollipop. It achieved this through a series of optimisations (e.g. a new battery saver mode, job scheduling APIs which restricted certain tasks to only activate over Wi-Fi, and performing tasks in batches to minimise the overall time that internal radios were active.

Of course, since that time Apple has also made improvements in how iOS manages power consumption.

However, in terms of answering your question Are these measures true, correct and verifiable? when referring to an iPad still displaying 100% after being idle with Wi-Fi activated all night, the only way for you to know that in your exact circumstances is to do some testing, keeping in mind you need to ensure:


that you have the exact same apps running during these tests 
all apps (and iOS itself) must be the same version
the start and end times (and therefore also duration) are identical
the ambient temperature at the time was identical (in other words, you'd have to do this in the same air-conditioned environment set at the same temperature)


In a nutshell what you would do is something like the following:


Fully charge the iPad and disconnect it from AC power at 10pm
At 8am play a movie at full volume and full brightness
At conclusion of the movie, take note of the battery percentage
Play the movie again (still at full volume and brightness)
At conclusion of the movie, take note of the battery percentage again
Play the movie yet again (still at full volume and brightness)
At conclusion of the movie, take note of the battery percentage again (assuming you've still got a charge)


Now fully charge the iPad and leave it connected to AC power until 8am the next day, at which point you disconnect from power and perform the same test (i.e. same room, same movie, same volume, same brightness, etc).

Compare the differences in your battery percentage and determine how accurate that 100% reading is.

I suspect that in reality an iPad will use some battery power overnight with the screen dimmed off and WiFi connected, but that the usage will be quite negligible.

Only you can decide, based on your test results, how true, correct and verifiable the measures (i.e. battery charge indicator) are.