The /spreadplayers command treats entities on multiple teams as if they were all on the same team.
In the latest versions the command feedback also says that the spread distance is Infinity:
- Create two teams
- Summon two armor stands which are in the teams
- Use /spreadplayers for them
→ They were both teleported to the same position
Based on 1.11 decompiled using MCP 9.35 rc1
The start message commands.spreadplayers.spreading. ("Spreading ... blocks around ...") uses the entity count instead of the team count.
The methods net.minecraft.command.CommandSpreadPlayers.getNumberOfTeams(List<Entity>) and net.minecraft.command.CommandSpreadPlayers.setPlayerPositions(List<Entity>, World, Position, boolean) treat all entities which are not a player as if they were in no team.