Side hustle tax

I'm currently completing a self assessment for the tax year 23/24. I made £2,226 selling things online as a side hustle. As well as that I work another job which I get taxed on each month through the company I work for. However it is showing I need to pay £459 tax? I assumed as my side hustle earned under the tax threshold I would not need to pay any tax on this? I've spoken to HMRC on the phone and they told me they add all my incomes together and as it's over the tax threshold they will charge me 20% for my side hustle? Is this correct? It doesn't seem to make sense as I've already paid tax on my main income? Also I've checked articles and many web pages about the 'side hustle tax' which has come to light this year and this is not stated anywhere, so if this is correct it is going to catch a lot of people out.

Hi,
This is correct. your personal allowances are used against your main job so anything else will be taxed at basic rate.
Thank you.