Having killed many test animals could be about finding the limits. It certainly isn't a sign that the tech is poorly developed. (Tho another response says that the deaths were irresponsible)

At the same time, killing many test animals says something about the ethical disposition of neuralink. Not that they are unethical. But that they are not on the side of caution with action in the ethical debate, and more on the side of 'not developing life-improving tech is unethical'. Which is certainly a colorable ethical stance.