The "Hojo Horde"

[Irinami]

Well, I'm not not going to declare that it's a bug or not a bug.

I am: It isn't a bug.

Reviewing my Shogun: Total War (Warlord Edition) manual, page 40, under the Ports heading. Quoting the final 3 sentences, emphasis mine:

You should also be aware that any province with a port can be invaded from the sea. To do this, you must first infiltrate the province with an Emissary, Ninja, Shinobi, or Geisha. If they survive the turn you may use their inside information to launch an invasion force from your own ports.