They can do that. Contract allows for company to skip wage progression. It also allows for greater pay than what the contractual agreement was, as long as it’s applied unilaterally to everyone.
Conversely, the contract doesn’t address sign-on bonuses specifically. Therefore, they can give out a one-time payout to a new-hire if it’s part of the hiring procedure.
No one ever thought Teamster jobs would need hiring bonuses to attract drivers.