Hate to be that guy but even with "superior modern technology" in isekai, cutting through steel that thick and wide wouldn't be fast in the slightest. It'd be slow as fuck
Also even old oxyacetylene torches use a cutting lever. Unscrewing a "disconnecting valve" to blast out molten metal while holding a torch with one hand would be both clumsy as hell and completely fuck up the kerf, not to mention there's no way in hell you'd get that smooth of a cut going horizontally on steel that thick
"When you mix fuel, metal oxide and metal powder in just the right way, it burns at 2000 degrees Celsius. Hot enough to cut through nearly any barrier known to man. Throw some C4 into the mix, and you've got one hell of a combination." - A wise man