This is a great question. It’s like asking when a rock is too small to be a planet. I suspect there were be a definition eventually that mirrors the planetary definition – something like “spherical(ish) and clears its orbit”. The issue is that Mars would lose its two moons under that definition.
So we might end up with something like “moons” vs “natural satellites” and Mars will just have to suck it up.
You don’t need the event horizon, you just need local gravity around 1G. For the masses described in the article, that radius is from hundreds of meters to 10s of kilometers.
Which still wouldn't do what you suggest. The mass is the same, so it has the same effect from a distance. Unless by "eat earth" you meant it would take in dirt until it suck to the core, still about the same mass.
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