There is no radical change.
To repeat, in England & Wales, the Sentencing Council's guideline for young offenders and children says sentencing should focus on rehabilitation where possible. That is similar to what has happened in Scotland. It is not a radical change. It does not require primary legislation.
To say yet again, primary legislation sets out the maximum and, in some cases, minimum sentences. It may also set out some principles to be considered in sentencing but, contrary to your post, the Sentencing Act 2020 (which does not apply in Scotland) does NOT say that rehabilitation and punishment must be given equal weight. It lists them as part of the purpose of sentencing along with a number of other factors, but is completely silent on the relative weight to be given to these factors. It is therefore open to the Sentencing Council to decide on the relative weight, as indeed they do.
The Sentencing Council determines the starting point for sentencing, the range within which most sentences will fall and the factors to be taken into account. That is the same in Scotland as in England & Wales.
There is no "someone else". There is only the Sentencing Council exercising its statutory powers within the framework set out by legislation, which, as I have pointed out, is broadly the same in Scotland as it is in England & Wales.
In this case, the SNP were determined that young offenders should include anyone aged under 25, so this was pushed through despite widespread opposition in the consultation.