In an all total conventional land war, like world war two, its hard to imagine ISIS conquering Luxembourg.
ISIS may be nastier than Luxembourg. Pound for pound its also even nastier than China. But nasty isnt the same thing as powerful.
ISIS has maybe 50 thousand fighters. Luxembourg is an actual country. In an all out war Luxembourg could mobilize 80 thousand men, and the country has an industrial base with which to support a war machine. ISIS is a stateless actor trying desperately to become a state. They have seized a couple of medium sized towns, and control about 35,000 square miles of mostly desert . The parts that arent desert are third world agragarian societies that cant support a war machine.They have no air force, and rely on criminal activity to finance their war machine which relies imported weapons.
If ISIS wins against all of the warring factions in Syria (including the Russian backed Assad), and if it wins against all of the factions in Iraq (including the American backed Kurds, and the American AND Iranian backed Baghdad government) then ISIS might in theory control all of both Syria and Iraq. And then ISIS could attempt to weld both Syria and Iraq into one big functioning country. And then if ISIS tries real hard they might ALSO be able to attract investors from abroad (or somehow generate capital from within) to build up industry in Iraq and Syria so that that the two countries could actually make weapons, and actually have a rudimentary war machine. That might only take....oh...fifty years. And then after that then-maybe ISIS would become a threat someone beyond beyond Syria and Iraq.
After uniting Iraq, and Syria, and after decades of industrialization, then ISIS would be ready to strike east. To conquer Iran (bigger than both Syria, and Iraq combined), and then to keep on marching east- enslaving vast stretches of western and central asia ( Pakistan, former soviet republics like Uzbekistan, and Kazahkistan, and it would probably have to sieze territory in northern India. Then, and only then, will ISIS have reached the western borders of China!
So it would take all of the above future history to happen before ISIS would even be in a postion to attack China.
And then when they finally do march across China's western borders China's vaster army would just squash ISIS under its thumb in a heartbeat.
I don't think China is worried about ISIS 'conquering' it any time soon.
But China does worry about its Muslim minority in the northwest, the Uighers, and about individuals in that group allying with international jihadists and causing trouble. But terrorist strikes, are not the same thing as being subjugated by a foreign army.