Thanks for the chapter!
Seems a mistranslation again. LN describes in more detail but gist of what the goddesses are talking about on page 3 is that one of the seals of the Demon King that the Hero defeated 200 years ago was broken. Although if look at the seal as a whole rather than parts could say becoming undone when part of it is broken. In any case when Hero defeated Demon Lord the soul was sent to another world and the main body and power were sealed with multiple points hidden in various areas to anchor and sustain the seal. So the sword the two idiots found (along with warning not to pull sword out lest release a demon but felt demon rather than sword must be lonely so were going to free it,) was part of the seal until the sword was pushed into the ground breaking that part of the seal. Part about pulling the sword out was to trick any of the Demon Lord's minions who found the place into pulling the sword springing the pit trap while thinking that was the way to release the Demon Lord when reality was had to push sword in. More details will be explained later.
Also, LN included how MC is surprised sister went to Mt. Fuji because during a proposed family trip there she got extremely agitated and made a hell of a ruckus not to go but never explained why. That and sister has been looking into why MC suddenly ceased to exist except for her memory (nobody else remembers him and even his appearance in family pictures and videos has been erased, so a pic with him and her together now just shows her,) and the circle with writing in it is part of an advanced mathematical formula that covers the entire floor.