Most cements melt at a higher temperature than most lava gets to, so it would be solid chunks of cement getting blasted miles out when the pressure builds high enough to erupt

Yes. That's the joke.

Even if the plug would hold the volcano would just split open another hole in the earth and erupt from there

Yes, there are many levels of stupid in this person’s proposal.

edit: “heavy metal rocks” would likely also meet the same fate.