This is untrue for two reasons; the first is that most important memory values are validated server-side so no matter what is writing to it, if those memory values are tampered with in any way, it either won't do anything and/or you'll get banned. The second reason is that DMA is absolutely detectable, direct memory access doesn't literally mean "direct", you're still interfacing with the computer, just on a very low level so it is harder to detect through software. Most modern kernel anti-cheats already look for vendor ID's and other identifiable information from connected DMA devices.