I think it's important here to bear in mind what this means. This was a criminal trial, to the criminal standard of proof ('beyond reasonable doubt') and to the test used for gross negligence manslaughter, i.e. that the defendant was not just negligent but fell far below the standard expected of someone in his position. No doubt the judge will have directed the jury in terms similar to those approved by the Court of Appeal in the 2016 case R (on the application of Oliver) v Director of Public Prosecution:
"A proper direction to the jury on the issue of gross negligence was held in that case to be that they should be sure that the conduct in question was something ‘truly exceptionally bad and which showed a departure from the standard to be expected’ so as to constitute the very serious crime of manslaughter. The bar is thus set high: perhaps unsurprisingly so, given that such cases ordinarily involve no criminal intent."
An acquittal means that the jury were not satisfied, beyond reasonable doubt, that the crash was caused by Hill's flying being so bad as to meet that test.