Obviously it doesn't make sense to run on 5G permanently because energy consumption is still comparably higher, just like it was the case with 4G in its beginning, but the automatic switching does a somewhat good job at it.
5G also has a ton of other benefits such as being able to support a multitude more clients and being able to prioritize traffic (like for emergency services) and handle slicing/QoS much better.
Not to mention stuff like e.g. active beamforming capabilities reducing power consumption and lowering emission levels for the general environment SIGNIFICANTLY.
All of the amazon internals and obvious marketing/hype around 5G aside: Author is a good example of naysaying - Saying no to obviously better technology because it doesnt fit their reality and/or knowledge.