That's complicated.
A lot of uber-modern slang originated somewhere in black and/or queer culture.
You've heard the phrase "spilling the tea" or just "tea" in relation to gossip, I assume? It's ubiquitous now, but that's a direct import from the New York drag scene.
Cake in terms of (male) butts started in queer spaces, made its way to black spaces where it's been largely used to reference female butts, and has started to move into what's called "stan culture" spaces.
Stan spaces are small, specialized, insular groups of people who are obsessive fans of a certain thing - could be a TV show, a movie, a musician, an actor/tress, etc. They tend to develop their own specialized patois of slang, much of it pulled from other groups, most often black & queer groups.
So.
Isaiah using the terms cake and double cheeked-up isn't some sort of subtle coming out, but the way they were used was enough to make me raise an eyebrow. He's either:
1. Heard them used so often in a space where they're used to reference male butts that it's "normal" to him
2. Heard them used in a stan space often enough to use them, though is awkward enough to use stan patois on a public-facing vlog not geared towards that stan group
3. Heard them used in black spaces and, without a trace of irony, went ahead and picked that slang up, used it in the wrong context, and failed at being "cool."
And thank you, again, for coming to my TED Talk.