Was the terrorist at the weekend a terrorist before or after he decided to go out and kill people? Obviously it was afterwards but the Police can only prevent such things by gathering evidence and they have powers to arrest people who refuse to comply with gathering that evidence. Robinson fits into the latter which is why he was prosecuted.
The real question is what were the Police expecting to find on his phone and why? The CPS must of had a convincing case or else they wouldn't have charged him. We are not privy to what they know nor the intelligence that the security services have on him.
Will Robinson go on for example to help instigate and fuel riots again? Robinson is somebody who constantly says that there will be civil war on the streets. His backer Elon Musk has said that himself. Who knows what is on his phone and what is being planned but nobody will ever know without looking and that's perhaps the point.
The offence is classed as minor and tried in a Magistrates for a reason, it's the bare minimum of what the Police can do. It was never going to make him a terrorist but perhaps he was suspected of being involved in something. Totally understandable given his history. The judge however wasn't convinced and well that just means that there's an absence of evidence but not necessarily an absence of reason.
Btw - attempting to overthrow the government, instigate civil war or instigate riots for a political cause does potentially class you as a terrorist as Palestinian Action have found out.