The DMA issue was only a factor for the disk drive. While the PCjr graphics memory was shared with the main CPU memory, the Tandy graphics memory was separate

The DMA issue was only a factor for the disk drive. While the PCjr graphics memory was shared with the main CPU memory, the Tandy graphics memory was separate in the same way that the EGA graphics memory was separate---it was never used for program code. In fact, the Tandy graphics was probably more compatible with the normal PC graphics than with the PCjr graphics. Yes, you could still access CGA graphics on the PCjr via $B800:0000 but the graphics chip rerouted the accesses to low memory were it was actually stored. But to use the 320x200x16 graphics mode (PCjr and Tandy supported this, CGA did not) on the PCjr, you needed to not only set the mode, but know the physical address (which could be anywhere in the lower 128K of RAM). When I wrote programs that used this mode, I had to use a custom MS-DOS configuration that set aside a block of low RAM for this mode. On this Tandy, you could avoid this and use the RAM set aside for graphics (starting at $A000:0000 if I recall correctly).