I think you should mention that in the case you describe, a direction for the solution has already been chosen (a SOA architecture) so you don’t need to go through a process of selecting the direction of the solution first.
That’s very valid, “The solution must consist of an off-the-shelf ESB product” is a perfectly valid requirement (constraint) if it helps to satisfy the needs of a stakeholder. It also represents on externally observable aspect of the system, so even though that requirement is a “design” (a ‘how’), it can still be a good requirement.

Erwin