This. There is basically zero chance that an ethics review would see this as a case in which a control group is unethical. I have been through plenty of IRB processes.

Resource constraints are a much more plausible explanation - running the study with a control group would cost twice as much (in time as well as money). This is either a pilot to apply for funding to run a better-controlled trial, or it's just not scientifically serious. Both are very plausible, in my experience.