Boeing is literally caught between a rock and a hard place. Their failure to adhere to quality standards in pursuit of financial gain has lowered their authoritativeness. However, they are experts and who should know whether a proposal is a wise option but the experts working on designing, building and developing aircraft? The concerns they raise are certainly valid and not inconsequential.
It is inevitable that these concerns will be seen as a spoiler and there is almost certainly an element of that in Boeing's comments. This has to be accepted as part of the environment Boeing and Airbus operate in - it is highly competitive and if one can legitimately delay or even scuttle a threat to one's market share then one should. One would certainly expect Airbus to do exactly the same if the boot were on the other foot.
This is not a good
PR move for Boeing but it is one that needs to be made - even though their motives might be suspect they are still raising what is a valid safety issue and doing aviation in general an important service.