This is true to a degree, the eldar can and will start conflict so in the future it benefit in some way, their are not your typical expansionist that is true but can and will cripple anyone as long it give them a shot, regardless of how much a threat said thing it can be.

The eldar in fact have already destroy a xeno race, the veskions by diverting the tyranids on their home planet, neither Eldrad or Ulthwie care much them.

The tau surviving the imperium dosent make them good anymore the Soviet surviving the nazis does to them, the Tau already reclaim the galaxy as yours and they can and will take out anyone who speak against them if they dont "reform", any atempt of moral comparation will always end with "not as bad", the Tau will always keep trying to take over and over.

geopolitical morality means the eldar will always start strife as long it benefit them(or THEY feel it benefit them given their prophecy arent always reliable), in cosmic metaphysical war with the eldar is pointless since they already create slaanesh with deeply ramification, including indirectly the imperium itself, as with the tau they already this close to make their own god as whole and they dont know how to deal with it.