Could a phrase "Don't buy from X" with indirectly implied material be considered defamatory?
No. The phrase is only an order, wish, or instruction, which is different from making a direct or veiled statement of fact. Even in jurisdictions where a statement of opinion could be actionable, a stated wish of that sort cannot be construed as defamatory.
anyone that reads the marketing material would indirectly have a false picture about company X.
It depends on the contents of the marketing material, on which you did not elaborate. Persuading the customers on the basis of truthful representations as to why X's product is better than A's does not mean that people have a false image about X.
The context and exact wording of someone's statements could amount to omitting and/or juxtaposing facts in a way that conveys some falsity, but your description gives no indications of that being the case here.