How would an acceptance of the validity of SSPX marriages be an attack against the traditional doctrine of marriage?
You're missing the point. The heretics in Rome are saying they will only recognize an SSPX marriage if they themselves, the Roman heretics, preside at the ceremony. If all priests were beamed up tomorrow we would still have valid marriages and baptisms, you don't actually need a priest for either, so heretical Rome has no right or authority to declare SSPX marriages invalid.
Now that Rome has meddled with SSPX confessions and marriages, will baptisms be next?
Clever plan of attack for them, don't debate the tenets of the Faith, but undermine the sacraments, the modernists work just like sappers did when besieging a castle; come in from the side and work towards the middle.