Who is responsible for delivery in Agile?

In discussions like this, it is helpful to distinguish between Accountability and Responsibility - as used in a RACI chart. Every task on a project has one or more people who will do that work, and they are all considered Responsible. However, for each task there can be only one person Accountable. That one person is the one who is held to account for the success or failure of the work.

Regardless of what the standards say, human nature is still human nature, and the PM will be held accountable and blamed if the project does not go well. This is not entirely unreasonable. We WANT the PM to be considered the expert on project management, and to be capable of marshaling the support and resources necessary for project success. If the PM is put into an impossible situation, then it's up to the PM to say so, to let the project sponsor know that the project is not structured or supported for success.

If you find yourself in an impossible situation, then the PMI Code of Conduct provides the answer. Either it's impossible because you are not a good fit for the project, or the environment is not conducive to successful projects no matter how good the project manager. Either way, the guidance is to not accept such roles. Hold your ground, and find other employment if necessary, or else your reputation and that of the project management discipline are at risk.

If I sound like I'm coming on strong about this, it's only because in hindsight, there have been times that I wish I had taken a stronger stance rather than hope that it will turn out, and I have learned from such experiences.