Errrr, who?
Anyway, It'll come down to the day, and who plays better (obviously) but Millwall are 6/5 and Swindon are 12/5 for a reason. We would have easily got automatic if we didn't have one of the worst injury streaks I have ever seen in the first 1/3rd of the season.
Millwall are the far better side, but playoffs are always a lottery. Anything can happen.
The odds you state are for winning in 90 minutes.
Overall winners odds Millwall 1/2 Swindon 11/8, roughly the same odds offered for Cardiff v Blackpool.