Warning: I’ve edited the comment that you’re replying to. I’m saying this for the sake of transparency, as you’re clearly quoting the earlier version.
The key here is that AAVE is not written, but AAE is. That “V” is for vernacular, it excludes written English by definition.
Now, I’m not sure if those white kids are using AAE or simply borrowing things from AAE into their written English. I simply don’t have data on that.
There’s been bleed over for centuries, but with the Internet and social media it’s merging faster, which is common for dialects of people that interact frequently
Varieties merging or splitting is rarely the result of just more contact between people; it’s all about identity. If things are happening as you described them, it’s simply that those white kids stopped seeing black people as “the others”, to see them as “part of the same group as us”.