> Did those stages fall uncontrollably or was there a de-orbit burn?

I did a bit of digging and found that a total of six S-IV stages launched by NASA using the Saturn I in 1964 and 1965 were left to make uncontrolled reentries (each with a mass of about 10 tons including attached payloads). Likewise, the S-IVB stages from the Apollo 5, 6, and 7 missions were left in orbit to make uncontrolled reentries (with masses of 10 tons or even more when residual propellant loads are included).

After that, NASA made sure to safely dispose of the spent Saturn S-IVB stages because of the hazards of uncontrolled reentries. For the three manned Skylab missions as well as ASTP, changes were made to the S-IVB plumbing and operation procedures to deorbit the spent S-IVB stages over the Pacific to minimize the risk of falling debris.

> Even then, are the Chinese really contempt to be compared to USA in the 70s?

Despite the fact NASA was fully aware by 1973 of the hazards of the uncontrolled reentry of large stages (which is why they started controlled deorbiting spent S-IVB stages in 1973), it was deemed not worth the expense and effort to modify the S-II second stage of the Saturn V to safely deorbit the spent stage for a one-off launch of the Skylab station. NASA decided to accept the known risks... just as the Chinese have decided to accept the risks of uncontrolled reentries of the LM-5B core stage today.