Sure, the message written on the bus could have gone into more detail, but it would be a bit ridiculous to put small print on the side of a bus. The point is to have a short, snappy message that quickly gets a point across and can be read from a distance. Further details were available from the website, on leaflets, etc..
http://www.telegraph.co.uk/content/dam/news/2016/05/14/97905...
I do wish that poster had been worded better. I'm sure it was a genuine mistake and not an attempt to mislead.
But hey, anything goes when there are votes to be gotten, who cares about facts and accuracy anyway?
Whatever the pledge was, people would have nitpicked and claimed it was a lie. There's no way to succinctly provide enough details on the side of a bus to satisfy everyone.