The prosecution argued for common assault in the appeal. To prove common assault, they would have to show that Grey's conduct caused Mrs Ward (the cyclist) to think she was going to use unlawful force and that the threat of force was intentional or reckless
To prove recklessness, the prosecution would have to show that Grey was aware of the risk that Mrs Ward would think she was going to use unlawful force, that she nevertheless decided to take that risk and that, in the circumstances, it was an unreasonable risk for her to take.
The Court of Appeal found that, taken at its highest, the prosecution case was that Mrs Ward may perhaps have feared a blow. That is not enough for a conviction for common assault. The prosecution would need to convince the jury beyond reasonable doubt that Grey's actions actually did cause Mrs Ward to fear a blow. "May perhaps" is nowhere near good enough.
Even in the unlikely event that the prosecution could overcome that hurdle, they would then have had to look at Grey's state of mind. Given her disabilities and cognitive issues, it is unlikely they would have been able to convince a jury that any perceived threat of force was intentional or reckless.
For a conviction for unlawful offence manslaughter, the seriousness of the offence that caused the victim to die is irrelevant - any intentional unlawful act that exposes others to a risk of harm is enough. Common assault, if proven, would have been enough to sustain the conviction. The issue was that there simply was not enough evidence to convict Grey of common assault or any other offence, and therefore she could not be guilty of unlawful act manslaughter.
So no, it is not that the acts were not serious enough. And the Court of Appeal acknowledged that Mrs Ward died as a result of Grey's actions. They specifically said that her death was a tragedy and the circumstances were horrific, and they acknowledged that Grey's actions contributed to her death even though they did not constitute a criminal offence.