It’s a masterwork. Some advertising genius was asked to encapsulate Joe Biden in a single phrase, and they thought hard about it, and distilled him down to two words.
It’s perfect! You see “malarkey”, and you think “antiquated, old-fogeyish” and it’s a natural to associate that with Biden. It’s going to go over well with timid old boomers.
In a particularly nice touch, in smaller print on the back of the bus, it includes a dictionary definition of “malarkey”. I guess that’s their idea of outreach to the young folks.