Their argument is that because he did not use the exact word "support" in respect to the Constitution, that he is not able to be excluded from holding office in the US even if he did commit seditious acts. He is saying that his oath to "preserve, protect and defend the Constitution" is entirely different than an oath to "support" it. It's nonsense, but one judge (in Colorado, I believe?) has already provided legitimacy to that argument, so... the stupid argument now has judicial precedent.
Edit: Correcting my mistake about the Judge's verdict. The judge did not uphold the argument that the Presidential oath was not to "support" the Constitution. Instead, the Judge was convinced by Trump's team that the President is not an "officer of the United States". Therefore, Trump took no oath as an Officer of the United States, and, thus, Section 3 of the 14th Amendment (which exclude someone who swore such an oath, who then incites an insurrection from holding federal or state office) simply doesn't apply to someone who has only sworn an oath as President.