I haven't seen any of this series. The last time I watched GBBO was when our local girl Cathryn Dresser was in it, which must've been 2012.
So this is only what I've heard, rather than my opinion.
The person who won was effectively lined up from the start to be the winner. The BBC had been accused of being too "white middle class" with their contestants. They needed ethnic minority finalists, and they needed an ethnic minority winner.
She baked really badly at the start of the series, but the BBC needed to keep her in, so the presenters said they could see some sort of promise in her, kept her in, and sent home contestants who had baked much better. Even if she had a bit of a disaster, she was still kept in.
In the meantime, some people within the BBC, who knew she was being lined up as the winner, made the mistake of placing too many bets. The significant betting bias in favour of the eventual winner aroused suspicion, and has led to the current inquiry.
In the final, she made an uninspiring lemon cake of some sort, and wrapped it in a patterned cloth, so the decoration on the cake was not done by her, it was just fastened around the cake. Because she had to win, the judges said that it was the best lemon cake they had ever tasted, or something.
The other contestants needn't have bothered turning up. They were always going to be runners up, even if the winner had used an Aunt Bessie's cake mix. The bookmakers had already realised that something was amiss, and had pushed their odds on the lady winning out to 1/7 ... but people were still betting on her. Nothing the bookmakers could do was going to stop people backing the known winner. She was the ethnic minority female, whose son was watching from a hospital. There was only going to be one winner.
Anyway, that's the word on the street. Personally, I know nothing.