Yes, train is cheaper than plane if you tack on an extra hotel night for no reason.
There's no difference between leaving on a Friday night and arriving on a Saturday morning by night train, and leaving on a Saturday morning and arriving on the same morning by plane.
Add to this that sleepover trains aren't available for the vast majority of destinations in Europe, and this article feels more and more like an ad paid for by the companies, which basically represent the entirety of the sleeper train option.
Addendum : I love the "a family of 4 saves £94, vut an even larger family could save up to £300!" part. Yes, if I took my entire extended family with me, we could even save over a grand on the trip !
"You’re also less likely to get stranded by a last-minute flight cancellation or lengthy delay - a nightmare that can destroy the most meticulously-planned holiday." Ah yes, the train, a mean of transportation that is notoriously known for it's reliability and lack of delay.
You want a cheap, low-impact holiday transportation? Coaches will get you across Europe for actually cheaper than planes, no ifs or buts, and aren't too polluting as long as they're full (also, trains are only "clean" transportation as long as they run on electricity and said electricity isn't produced by burning coal or oil)