Still just exported models, textures and geometry with barely any gameplay related code.
(even quake1 could get very close to this, ports even closer
)
A nice proof of concept anyway, but this is still lacks the essential elements which is a fully working placeable portal with render trough, the hardest part sadly
.
I guess one could do some clever texture that is actually an camera pointing from the exit (Kinda like how duke3D cameras worked), updating at a certain rate.
Good thing that portal1 was rather simple so not THAT much gamecode is needed to get much of the same feel.