Not strictly true -- they haven't taken the Oath of Office but they are sworn as individuals. Each representative-elect has sworn that they are legitimately elected under the laws of their respective states. That's a perfunctory, mostly symbolic procedure that was conducted in December. I believe the 20th Amendment alerted the mechanics slightly. They are functioning as members of a sworn legislative body but that body is not the 118th Congress.

There's a mountain of details behind this, partly in the modified version of Roberts Rules in use for parliamentary procedure in the house, and partly in the House Rules that have been continuously maintained since the first Congress in 1789.

Remember the January 3 session of the 117th Congress when the first order of business was to elect the Speaker (Pelosi), and the very first motion after she was sworn in as Speaker (but not yet as Congresswoman), Chip Roy of Texas as a member-elect made a challenge to the seating of members-elect from Arizona, Georgia, Michigan, Nevada, Pennsylvania, and Wisconsin.

That request was not even acknowledged, though in hindsight we should recognize it as the first official act of the insurrection. HR-1 passed with 371 yeas. That alone was likely an event without precedent. A full roll call vote was demanded just to affirm the Speaker's authority to administer the oath of office to all Representatives-elect. HR-2 assigned the House officers without objections, which is a surprise in hindsight. HR-3 communicated to the Senate that a House Quorum had been assembled, and HR-4 created a committee to inform the President (Trump) of the quorum.

The assembly of Representatives-elect to the presumptive 118th Congress has not even reached the first step of the process that the 117th Congress had concluded by lunchtime on January 3.