Taken out of context, that sentence loses significant meaning.

We argue that this procedure gives the President—acting through the FDA and DEA—power to reschedule marijuana to a less restrictive schedule (as the Biden Administration is currently trying to do). Those administrative agencies can conclude that marijuana has an accepted medical use and a relatively low potential for abuse—characteristics that align with placement on Schedule III, IV, or V.

That’s the part you skipped, which indicates that the president has the power to direct the agencies to reevaluate and reschedule. They further contend that this process could be used to entirely unschedule the drug.

The question being answered in the portion you cherry picked is not if the president has unilateral authority, but rather what the extent of the administrative process they must follow actually is.

An even more direct segment from the CRS report:

If the President sought to act in the area of controlled substances regulation, he would likely do so by executive order. However, the Supreme Court has held that the President has the power to issue an executive order only if authorized by “an act of Congress or . . . the Constitution itself.” The CSA does not provide a direct role for the President in the classification of controlled substances, nor does Article II of the Constitution grant the President power in this area (federal controlled substances law is an exercise of Congress’s power to regulate interstate commerce). Thus, it does not appear that the President could directly deschedule or reschedule marijuana by executive order. Although the President may not unilaterally deschedule or reschedule a controlled substance, he does possess a large degree of indirect influence over scheduling decisions. The President could pursue the appointment of agency officials who favor descheduling, or use executive orders to direct DEA, HHS, and FDA to consider administrative descheduling of marijuana. The notice-and-comment rulemaking process would take time, and would be subject to judicial review if challenged, but could be done consistently with the CSA’s procedural requirements. In the alternative, the President could work with Congress to pursue descheduling through an amendment to the CSA.

What, exactly, do you think that says? Because I’m pretty sure you just read that as, “The President can tell the DEA to reschedule cannabis,” and that’s not at all what it says.

That paragraph you just quoted said it MAY be possible… but only if you completely reinterpret one law, use a wide interpretation of how we deal with treaties, and also just ignore other laws in deference to the President.

So basically it is only possible if we ignore the law.