DOS emulation is not a very battery-efficient way to play '88-98 games on the go, so projects like these seem to cater to that specific need, letting people run DOS natively. The Toshiba Libretto was a nice, small machine, but finding one in good condition is super hard. Toshiba Portégé was another, but after Pentium-II models, they took away the OPL3 card in favor of something more Windows-friendly (AC97), which doe…
They don't mention the battery capacity or specific 8086 variant used but modern ones use to average 2W of consumption; there are zero power saving modes; and the guy even says that when using 8087 the mini-laptop must "be plugged in all the time".
So I rather doubt real hardware is the most efficient way, specially when e.g. with virtualization you can easily have 20+ hours of DOS on a subnotebook.