Even in the original this wasn't allowed, I think it might be because there are no meat present in the craft to provide instructions to the tanks :p
I don't think the soldiers are who is providing the instructions.
It is likely that the development team would have needed to make a redundant set of code or re-write existing code in order to have enabled tanks to go to the mission alone. To that end, they would have:
1.) denied it, considering it unworthy of development time or code space
2.) placed it on the backburner and never got around to it
3.) maybe didn't realize this would happen, limited playtesting by a small number of people may have never happened upon this situation
I doubt that they felt it would be a good idea for a base defended only by tanks to never get a chance to fight the aliens. While it is possible any of them felt that a mission shouldn't be only tanks, they probably would have only supported such a feature if they weren't aware of the hardships potentially endured by enforcing the rule.
But this is openxcom, the devs of this project have decided there is no reason to include this bug/feature in the game, and thus we have the capability of doing the following things:
1.) creating a tanks-only mission in the battle generator, any type
2.) defending the base with only tanks
3.) going to a landed or crashed UFO with only tanks
like yataka shimaoka said try the del and create again if you havent done it already
crafts:
- delete: LIGHTNING
- type: STR_LIGHTNING
Why would that work? It shouldn't matter whether the craft is in the original ruleset or not.
I just did some testing, this time only using craft that my mod adds to the ruleset. I was not able to get the Stormstrike or Vanquisher to land at an alien base with only tanks, but they will happily land at a landed or crashed ship with only tanks. It would seem they are unwilling to land with only tanks on terror sites and alien bases only, and I don't see any pattern as to why these sites would be treated differently except that both use a magenta tag on the geoscape.