For one, I’m not sure the quote means that we should seldom use words, but that we should be “preaching” by our orthodox behavior even when words aren’t appropriate.
Secondly, his quote is just a little more fun and ironic.
Thirdly, the quote may not have been his exact words, but your version doesn’t really fit the entire story from which it comes:
"St Francis received several new brothers into the order. Wanting to see for themselves their father’s public preaching (as the Saint was wont to do) they asked for permission to accompany him on his next trip to the town.
The Saint agreed, and called them one day to accompany him. As they walked to town, their father led them in many hymns, joyfully singing praises to God. Along the way, Francis begged for love of God among the rich. They stopped in the parish church, and gave homage to the Eucharistic Lord. As they entered town, he gave of all that he had begged to the poor. Lepers greeted him in the home he established for them, and he lovingly bathed their wounds, kissed them, and turned to return to their house.
As they returned along the road, the brothers asked the Saint when it was that he would begin his preaching?
He replied, “What do you think we have been doing since we left? I tell you that we are to preach the Gospel always, but only use words when necessary.’”
Fourth, this story predates the liberal theology you referred to.
And lastly, this story, itself can’t be documented and proved beyond doubt, but neither can many things that we accept through tradition (little “t”). But gosh, why ruin all the good stories just because we got a point to make. I mean, who wants to start telling elementary school kids that when George Washington go caught cutting down the cherry tree, his answer was, “It seemed to have some sort of parasite, Dad, and wanted to make sure it didn’t infect the other cherry trees.”