> Perhaps he did it because heat gun would have impacted other parts of the PCB
Most of all the components on the other side of the PCB.
Even if the NAND was not additionally covered in resin, due to the large amount of contacts (and probably large amount of grounded pins, which dissipate heat faster), alot of heat would be needed. More than the smaller components underneath need to just fall off then...
The components underneath shouldn't fall off. The surface tension of the solder should be enough to keep them in place. (That's how it's generally possible to reflow solder components to two sides of a board.)
Of course, reflowing the components in an ad-hoc way is quite likely to cause other problems, so it's probably still a good idea to avoid it.
Most of all the components on the other side of the PCB.
Even if the NAND was not additionally covered in resin, due to the large amount of contacts (and probably large amount of grounded pins, which dissipate heat faster), alot of heat would be needed. More than the smaller components underneath need to just fall off then...