Assuming the Nihilanth isn’t a one-off creature and does have other beings like it in existence, I think it’s possible that the other Nihilanths were used by the Combine for local teleportation.
Forever in the vortessence you will be remembered for times to come and times to end our hearts will never be sallow and you shall be the highest among the vortessence known as the one and only Louis Goussett Jr.