Eh, you can always head-canon it as just a regular truck if you prefer it. The film as it is leaves it open to interpretation which I think is cool.
When I first saw the truck driving across the bridge I just assumed it was a regular truck that happened to be coloured like G1 Optimus Prime as an Easter Egg. Then I saw the forest scene where Optimus Prime talking to Bumblebee, and that concluded for me that the truck was indeed Optimus Prime. But if people prefer to interpret it differently then I see nothing wrong with that. Many of us have head canon interpretations which may be different to official lore, but IMO as long as your head canon doesn't directly contradict official canon then why not?
P.S.: Transformers The Basics on Blitzwing just came out. McFeeley points out that Bumblebee Blitzwing is Blitzwing in name alone but is otherwise just another Seeker. Still... loads more character presence than most Bayformer 'Cons!![]()