That whole argument is a non-starter.
If you're going to claim/imply that the lack of mention about particular speeds creates any ambiguity in the contract terms, you should keep the following in mind: any ambiguity in a contract is, by law, resolved in favor of the non-drafting party. So, if the lack of specificity about the network speeds creates ambiguity, then the *user* of the plan, not the owner of the network who drafted the contract, is the one who benefits from said ambiguity.
----------
Are you claiming that the contract is ambiguous about the speeds offered? If so, you should know that according to contract law, any such ambiguity is to be resolved/interpreted in favor of the non-drafting party. So, if the specified speed is ambiguous (as you seem to be claiming), it actually *doesn't* leave room for the party who drafted the contract to interpret that however they wish.
----------
And those unlimited contracts have been extended by AT&T time after time when people renew. They don't offer those terms to *new* customers any more, but they continue to offer those terms to existing customers who signed up under those terms while they *were* available to new customers.