I think that Romanides' reference is that Leo consented to the Franks' desire to sing the Creed in the Liturgy *at all.*
The addition of saying or singing the Creed during the Divine Liturgy is a relatively late addition. Its ancient use was in baptismal services.
At least that is how I read Romanides -- Leo gave permission to sing the Creed in the Liturgy, but insisted that it be in its original form. I may be wrong, but I can't make it make sense otherwise.
Agrarian, Pope Leo III consented to filioque being sung but not inserted into the Creed because the Creed cannot be altered (in writing). However, every source I looked at, including Romanides', leaves no doubt that +Leo stated to the effect that filioque was theolgically sound and that he agreed with it. As to why he agreed with its theology is a different story.
Romanides has his own theory, which very plausibly takes the political situation and the reality of Frankish barbaric mentality into account. Others simply read nothing into it, nor try to paint the picture in the context of times, but just state tat Leo agreed theologically with filioque, while proscribing its insertion into the official Creed.
Romanides makes a valid point when he says that +Leo III specifically explained to the Frankish legate that the Fathers and the Councils did not "miss" inserting the filioque as a diplomatic way of saying he in his heart did not believe it to be theologically sound but could not afford to draw ire of his Frankish protectors.
I tend to side with Romanides, because contextual understanding of historical events is indispensable. However, the true reaosn remains unknown and only subject to educating speculation.