The "Law" is the Mosaic Code but also the Scriptures of Paul"s day, the Old Testament, particularly the legal parts of it. Paul probably did not mean that these erring teachers failed to understand the letter of their content, though this may have been true of some of them. He probably meant that they did not understand what they were really saying and not saying by their emphasis. They missed the point of the Law. [source][source][source]
Their "main interest seems to have been to rival contemporary Rabbinical exegesis, rather than to expound the gospel." [1][source]
". . . Paul"s description of their "confidence" implies in this context stubbornness, a refusal to be denied. We might say they are dogmatic, which (along with the claim to authority) Paul regards with irony, since they have no real understanding of the matters they teach." [2][source]
"This "apostasy" on the part of both the erring elders and their followers is the great urgency of1Timothy." [3][source]