The author's DEN-LHR flight was on a Boeing 787 aircraft, which use electrically-powered air compressors [0]. They do not draw air from the engine.
787 maintain internal air pressure equivalent to 6,000 feet of elevation. All other commercial aircraft maintain 8,000 feet air pressure. I think most of my discomfort from long flights is due to altitude sickness. The only time I have slept on a long flight and awakened refreshed was on a 787.
Jet engines use a neuro-toxic lubricant. Occasionally, some of this enters the cabin through the compressed air feed and poisons the passengers [1]. 787 passengers don't have this risk because the planes don't draw air from engines.
[0] https://en.wikipedia.org/wiki/Boeing_787_Dreamliner#Interior