I belive the Gaurdians were forerunner constructs similar to seekers used to combat the flood . Basically specially designed seekers on steroids. Maybe the didact made them and they can pump out promethians as well with built in composers . But anyways I think they are appearing because of they have detected a large flood presence. Cortana has said the reclamation has begun which means the flood will come to test us.
The. SOF had flood on it so I belive the flood have finally emerged and may of took control of several outer colonies, and are spreading without the UNSC knowing. Except for John and the bunch. The Gaurdians have been destroying planets that the flood are spreading too and just defending themselves if attacked. Some what to humanity cleansing planets in the forerunner saga and being mis took for attacks.
The planet Cortana is talking about Is where the covenant found the luminary, which lead them to the ark. The flood never knew this except for h3 so they may be trying to find the ark so the halo rings can’t be fired,and they will succeed. The other forerunner vehicles in the art shots are either from a full on forerunner force that the Gaurdians have sommuned. Or Jul madam found the absolute record but I doubt this due to the newer design of banshees which are madams combating these so I belive they are actual forerunners