iMessage is very different from Whatsapp. For starters, there is no dedicated iMessage app. iMessage is managed from the iOS Messages app, which handles both iMessage and SMS. It's very good at managing both from one unified interface. In fact, it's so good that it uses arbitrary coloring of messages so you can see what message was sent through iMessage and what message was sent through SMS, because you would often not be able to tell otherwise as it's all in the same interface.
A lot of caveats apply here, but broadly speaking, while an iMessage user is typing a message, Messages will contact Apple to check if the recipient is also an iMessage user. If yes, it will try to send the message through iMessage and color the text bubble blue. If no, it will go through SMS and the bubble gets a green color. If iMessage fails in the first scenario, Messages will offer to revert to SMS.
So unlike Whatsapp, Messages doesn't care if the recipient has iMessage or not. And it also does not care if the recipient has an iPhone or not. It will get the message over regardless, as long as the recipient can receive SMS, which any Android phone can. Whatsapp, OTOH, does put the onus on the recipient to have the app. In other words: when it comes to the recipient, Messages (the app) is actually a lot more inclusive than Whatsapp (the app).
These HS teens aren't locking others out because they can't text their Android friends. They perfectly can, as OP from the Twitter thread also acknowledges, from the same app that they use to text their iPhone friends. They lock them out because Messages colors the bubbles green, which has become a social stigma in our high schools. Nothing more, nothing less. So it is not a technical limitation, and neither is it a cost issue, as virtually all wireless plans include unlimited (SMS) texts in the US. It is purely a human factors issue, which teens of all times and generations have been very prone to. As sad as it is.