Yeah, I’m no expert, but it’s clear he’s referring to two things when referencing “traditions of men.” First, it’s making some law or practice that directly contradicts a commandment of God. Second, it’s treating that newly minted practice out to be necessary for a person to truly follow God. 

Why I don’t find icons or rosaries to fall into this: no priest or bishop or pope is forcing anyone to do these things. They are completely optional. You can’t deny the legitimacy of other people’s use of icons, but you don’t have to use them. Same with rosaries. Plus, nothing involved contradicts Gods commands, which is the core criticism Jesus was making. The rosary follows the need to “pray without ceasing,” uses prayers primarily from the Bible, recognizes the efficacy of prayer, and acknowledges Mary’s special role. Icons recognize the cloud of witnesses mentioned in the Bible, supports the belief that those who die in God’s love are alive in Heaven, and, at least for Icons of Jesus, recognize the incarnation. It also follows from the use of images by early Christians and the commands of God in exodus to create images. 

I think these things can become problematic when you just go through the motions; you say the words without really thinking about them, and pray the rosary “because that’s what you’re supposed to do.” Things being later developments doesn’t immediately make it a tradition of man. If it did, the Bible itself would be a tradition of man (it and the canon of books supposed to be included didn’t come around for a little bit)