I'm not entirely sold about the "Octoling is an Inkling" theory. At least I'd be hugely disappointed if that were the case - one of the things of note to me about the whole Inkling vs Octarian lore in the series is how Inklings as a species evolved because Octarians were driven underground, so in a way they're essentially a persecuted race, which makes the idea of turning an Inkling into an Octoling sort of carry some unfortunate implications. The lore in Splatoon previously indicated that only female Octarians ended up "evolving" to take human shape while male Octarians remained primitive. Of note is also how it's implied that DJ Octavio essentially brainwashed Octarians through the music that plays in Octo Valley (hence why the Agent needs to wear those headphones), and in the second game they outright replace the Octolings' visors with shades to imply that they're straight up being hypnotized.
Also; the hairstyle of the male Octoling is identical to that of the regular Octotrooper. Meanwhile, the female Octoling's hairstyle is identical to the Twintacle Octotrooper.
This is just my guess but I'm going to assume that the test tubes might be trying to "evolve" Octarians to allow them to integrate themselves into society. I'm not sure what to make of the brainwashed Octolings though, maybe they're trying to save them from Octavio's control?
