That's a bit of a misnomer/accepted marketing lie. Phones (iPhone & Android) really only made that jump from 200-300ppi when they switched to OLED displays with 'pentile' sub-pixels. Instead of every 'pixel' having three actual OLEDs, or sub-pixels, there are five sub-pixels divided into two 'pixels'. This article is older but explains the basics:
https://www.oled-info.com/taxonomy/term/153/all
There are upsides to pentile OLED but the downside is you need higher PPI to mask the artifacts. That in turn means the devices needs to push a higher resolution to reach the same levels of visual acuity/sharpness as a true RGB display.
As others have pointed out, you are very unlikely to notice any difference on a 4K LCD the size of an iPad unless you have your nose to the screen. Meanwhile, the iPad is having to work harder every moment to move all those extra pixels that provide little/no benefit.
If/when the iPads switch to OLED there may a jump in resolution but it will depend on how Apple implements the OLED display.
2,388 * 1,668=3,983,184 * 3 [sub-pixels]=~11.9M sub-pixels
2,388 * 1,668=3,983,184 * 5/2 [sub-pixels per two pixels]=~9.9M sub-pixels