I can't find the COSHH on Dirt Lube products either. I would suspect that the degreaser is similar to most other degreasers which are mixtures of Butoxyethanol, surfactants and some salts. Degreasers works to remove grease but they don't remove metal...too much. Soak them for a very long time...we are talking months...and they might cause a problem. The amount of degreaser you've used - about 10 ml - isn't enough to fret too much about. The salts in the degreaser aren't there in high concentration, they aren't particularly hygroscopic and don't have much affinity for steel. If they are left there, I doubt that you'd see any problem. They certainly aren't going to dissolve the joints or welds. A saline solution - 7% sodium chloride - would take years to cause that kind of damage and these salts have orders of magnitude less affinity for steel.

Part of the reason that you rinse degreaser off is because you want to remove the grease. Think of it like washing dishes. You could just apply the surfactant - fancy name for soap-, wipe it around and let it dry. But that doesn't get the stuff off the plates. Same with degreasers. The grease just sits there on the chain and isn't removed.

The problem is that, like your plates, after you rinse the degreaser off, you want to dry the parts before you put them away. With bikes that's a problem because you can't get into the nooks and crannies where the water wants to hide. That's why I suggested chasing the water rinse with something that dissolves the water and dries quickly. Denatured alcohol or acetone does a dandy job. You could even chase the degreaser in your seat tube with some water, rinse with alcohol or acetone and let it dry on the - let's see you said it's winter now so I'm guessing the sun is to the north - north side of the house in the sun. If the bike is relatively new and has cartridge bearings you can probably do it without taking anything apart.

Use a scotchbrite pad on the rust spot by wrapping it around a dowel so you can use it to scrub at the spot inside the seat tube. Then cover the new shiny metal with grease. Grease your seatpost too.

In the future, I'd suggest using mineral spirits - aka naphtha, white gas, benzine (NOT benzene ) - to do all your degreasing of moving parts. Fewer steps, less volume, and less hassle.