Probably a visitor to the UK. It happens a lot with cyclists visiting the UK. They either don’t know the rules or they make a wrong turn and find themselves stuck on the motorway. IIRC the Kenyan national team (or some other national team if I’m mistaken) was using the motorway to train on in Manchester during the commonwealth games. Motorways can look very inviting because they have wide smooth hardshoulders.
In quite a few countries you can cycle on motorways, like in Serbia.

I wouldn’t automatically call the person a scumbag. Chill.