The reasons behind this statement go beyond a failure to understand what the job entails, which has already been adequately explained.
Boris does not get along well with the trades unions, particularly the RMT that represents the majority of workers on the Underground, and is attempting to polarise public opinion. By misrepresenting the complexity of a train driver's job and the amount and duration of training required before a driver becomes fully qualified, he is attempting to paint drivers as overpaid people doing a simple job so that any actions taken by their union with regard to pay and conditions appears unwarranted.
In any case, Boris is FACTUALLY wrong. I drive on the mainline railway and not the Underground, but the job is very similar. As a new recruit it took ten months to qualify and some drivers take even longer than that (we were rushed through about as quickly as could be achieved), not a few weeks. You have to learn the rules (like the Highway Code), traction, routes and then have a period of practical instruction.
The other thing to bear in mind when looking at driver's pay is that the job is very intolerant of mistakes, far more so than even politicians. If you accidentally miss out a station you can't go back and undo it. It is a professional job that requires 100% concentration at all times, often at very unsocial hours of the day and night. It also impacts massively on your home life because you have to manage your rest and be careful about things like when and how much you can drink.
Yes it all seems very easy when looking at it from the outside as Boris is. However, Boris's comments are more politically motivated than anything else.