This is interesting. So it did say a cast off row, right?
I think the instruction wants you to cast off knit wise but you still have to follow the pattern while you doing bind off.
I would do it like this;
K1, k2tog, then bind off, then knit and bind off the next 18 sts as usual, and then k2tog, then bind off, and then bind off till the end.
Does this make sense?
Thanks for sharing your question. Have a great weekend!