I think the reaction to the police statement in some quarters is a bit bizarre. It was JKR and her supporters who have been adamant the Bill was going to result in mass arrests for nothing more than stating fact. This has been disputed all along by those responsible for the Bill, who highlighted that the threshold for "stirring up hatred" is extremely high, and that Police Scotland should not have any trouble determining where and when the new aspect of the Bill might come into play.
All that has happened today is that Police Scotland have demonstrated that having received a complaint, they have proven they are able to make an appropriate judgement that nothing that was tweeted in any way constitutes a hate crime or "stirring up hatred".
How this translates into a "win" for the Bill's critics when it demonstrably proves their claims were nonsense I do not know. Nothing that was tweeted would have been a crime prior to 01/04, and evidently nothing has changed, despite the claims the Bill would usher in a new era of suppression of free speech.
The Bill does not criminalise anything that was not previously a crime, with the exception that "stirring up hatred", which has always applied to race, now also applies to other protected characteristics. If the Bill was going to result in mass arrests for "thought crime", then I think it stands to reason that we should have been seeing mass arrests prior to 01/04 for "stirring up hatred" with regards to race, because there is plenty of racially-based hate around, yet the threshold for that has always been so high it's a rarity.
You are correct OP, in that I do not expect the Bill to have significant material impact, but that was never the claimed purpose of the Bill in the first place. The claims about over-reach, dire consequences, and mass arrests were being made by the Bill's critics, not it's proponents.