S1 k1 psso (aka skp) is structurally identical to the SSK, and so mirrors k2tog well. Will these decreases be visible on the finished piece? If the will be covered by, say, a turtle-neck collar or similar, it might not matter if they don't match. But if visible, I'd definitely use SSK/SKP instead.
It could be that the designer doesn't care, or that she finds the k2tog tbl preferable because of her personal tension (some people can find that their SSK is enlarged, and the twisted stitches when knitting tbl give a tighter result).
This is one of the best left leaning decreases I have found. It’s fairly fiddly, but it ensures that you don’t have that extra loose bit of yarn in the top stitch.
The best solution I've found is to slip both stitches knitwise on to the right needle, put the left needle through the front of both needles, and knit them together through the back loop. Give it a try. It'll look just like the left side.
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https://yarnsub.com/articles/techniques/k2togl/