As far as I can tell only attacking to gain continents, or prevent someone else from keeping/gaining one, is called solid tactics.
Except you arent preventing each other from holding continents. When between you you held europe and asia (one each) and had a set of cards to use, you would have had ample units to attack the other and so stop them holding their continent. INstead you both attacked down into Penfold and left the other alone and so let them gain their bonuses next round. If that isnt an alliance, can you explain what it was.
This was Penfolds reason for reporting this one (as his regular doubles partner he askd me to look at this game) and it was pretty damn obvious they were not making any effort to disrupt the other player.
As they have confused they all know each other and i think the game log makes it clear they are concentrating on taking out the third player and so leaving them to fight out for the win. So, whilst not a multi, its an alliance. They may deny it but i think the log is pretty clear, and looking at another game they are involved in I'd suggest exactly the same thing is happening there.
Answer surely is to ban them playing against each other in any games, that way they cannot be accused of cheating.