Well no: not only the reset vector will be in the boot area, the interrupt vectors will be fetched there too. So if your program uses irq's this will not work.
actually it does work because ivt can be relocated from user code using ivce/ivsel bits in gicr.
anyway imo it dont look like usbasp, avrdude, or fuse bits prevent writing upper flash.
99% of random ebay purchase work out if they physically resemble the "good" type. generally baite assembly house with askew mcu. this used to be lowest cost for a dollar or two. now the other type with blue pcb and aligned mcu account for most cheapest.
... no problem with t13, m2560,
like before some problems did crop up after reflashed with fischle firmware
How do you tell whether your cheap eBay USBASP has "correct" firmware on it?
Ya gets what ya pays for.