Interesting write-up. Having been a bookkeeper a long time ago, I'm not too surprised at this being susceptible to automation by an LLM backed system.

It seems also that the classes of error they encountered could be handled by improved skills/knowledge base access on the fine points of relevant tax legislation.

The important part for their software ofc is, will they take responsibility for the output if HMRC come calling? Without that users are adopting the risk which they may not be keen to do (dealing with HMRC is not fun), with that it could be a very nice saving for a lot of small companies (and bad for the employees of a lot of accountancy firms)