Hi and welcome to KH, I had to remove the whole pattern 'cause of copyright laws. Can you give us the name of your mitten or a link, so we can help you better?
It wants you to work the 8sts 'cause it's setting up your increases for the thumb. It does want you to knit across 8 sts of the thumb, you will then turn so the wrong side of the mitten is showing and cast on 1 more st. Then purl back across, turn so the right side is showing and cast on more st.
You will then have 10 thumb sts and work in pattern until it's 2" less than the required thumb length