Heckler & Quiat will try to convince the court that the (entire) letter satisfied the formal requirements of a legal bilateral contract (i.e. offer, acceptance, and valuable consideration) in conjunction with the date and definite amount of value transferred between the parties. In effect, their position is that the first sentence of the letter was actually the first clause of an offer and also the first clause of a contract and that the second sentence was a second contractual clause specifying Jones' exact obligation under the contract.

Jones' position is that the meaning of the first sentence was simply a congratulatory announcement that Jones had already earned a severance package by his previous work. The second sentence was nothing more than an instruction...

In effect, Jones' position is that his previous performance at the firm should be considered past consideration for the severance package that he received in return.
It is likely that court will not accept Jones' interpretation. The court will probably characterize the letter as creating a contract. In that analysis, Jones would only have been entitled to the payment if he performed his contractual obligation by giving his full attention to Heckler & Quiat client matters for the duration of his continued employment. Since Jones did not do that, the court will consider that to have been a breach of contract excusing Heckler & Quiat from their obligation to perform by paying Jones after terminating him without cause.