I think you are wrong there. I expect most people who donate to RSPCA know nothing about horses and are anti hunting l. Many of their supporters may have seen that video and been horrified and the RSPCA could easily have been lobbied by their supporters to prosecute. They may even have high net worth supporters who have been willing to donate to fund the case.
The RSPCA has some very wealthy supporters including Deborah Meaden who has horses
https://www.looktothestars.org/charity/rspca
I agree there are certainly situations which are much worse including dodgy dealers and professional riders but because they are not seen by the right person they probably get away with it.
This is probably strategic they have to been seen to be doing something if they didn't they may lose supporters and they may even gain supporters from this case such is the vitriol against hunting from the average person. Think of it as a fundraising investment. Yes the case will cost a lot of money but it will raise the profile of RSPCA, may attract new supporters and stop supporters from leaving they could actually end up being financially better off by taking this to court than by letting it go.
I doubt they will win. If they do win it will be a wake up call for the equestrian community as anyone who is able to film similar sort of behaviour would be able to send it to the RSPCA and expect them to investigate.
What we don't know is if this was a one off incident where the stress of the situation lead to unacceptable behaviour or if this woman regularly treats horses like this. If this is normal behaviour for this lady then it is a very different situation than if it is a one off.