You know it could also have been gearing. Because I have a big granny on my road bike, I sacrifice upper gears. I can hammer but if someone has taller gears, I can get dropped. If you were matching stroke per stroke and then they pulled away, it maybe they were pushing a taller (bigger) gear.

And if drafting, you would know it. The ease of which you have to pedal will be considerably lighter, easier if in someone's draft, like you can coast and they still have to pull. If working as hard, you aren't drafting!