Where did these come from in C2 L5? Have they been covered previously?

They’re covered in Lesson 0 at the beginning of Course 2. It explains several very common southernisms that are used from Course 2 onwards. Course 1 used the “standard” forms so you would be used to those as well as they are most likely what you will hear in more formal speech e.g. on the radio.

Can’t remember when they first appear as an alternative but here’s a guideline:

Sa i is the south Wales form of the the negative form of the
present tense of the verb to be.

Dw i ddim yn mynd becomes Sa i’n mynd

Dw i ddim yn hoffi - Sa i’n hoffi
Sometimes you’ll here Smo
Smo i’n hoffi; Smo i’n deal

The other forms are:

So ti or Smo ti - You’re not
So fe/So hi or Smo fe/Smo hi - He/She isn’t
So ni or Smo ni - We’re not
So chi or Smo chi - You’re not
So nhw or Smo nhw - They’re not

Diolch yn fawr