Wanting to do early experimentation, that is currently only allowed to occur on animals due to safety concerns, on humans is definitionally unethical and especially dangerous. We banned it because it is unethical and especially dangerous in basically every case. There are literal decades of experience and piles of bodies supporting that point.
Obviously there are some exceptions, but whatever Neuralink is doing is not one of them. Their technology and any benefits it might bring is at best wholly speculative and would require years to reduce to practice. There is exactly zero compelling reason why they should be given a pass on human testing safety.
Suggesting that it would be acceptable to replace their animal testing with human testing as long as the volunteers were willing is about as “open and shut” unethical as you can get.