And here's the reply:
Direct connections are possible with monitor models FMT-DP535 and later. As for the TOWNS displays, because there is no sound input compatibility, when connecting the X68000 and the PC9801 series (and the like), for peace of mind (layzee's note: I still have little idea what 精神衛生 means in this context), use them without sound wirings (layzee's note: probably translated this part wrong). Also, if directly connecting to a TOWNS display, a few X68000 15khz games will be incompatible, and this problem can be solved by using the cable being sold.
So to answer your original question, my guess is that, for the monitors you are inquiring about (FMT-DP535/5351/536/5361/5351W/5361W/8311/8711), the cable isn't necessary but if you want full compatibility with games, then you might want it. In contrast, some monitors do require the cable (FMT-DP531/532/533/534/一体型TOWNSディスプレイ ＵＸ，ＵＲ).
Disclaimer: A reminder that I'm neither a Japanese language or Japanese computer pro so I will not be held responsible for my third-rate translations leading to your wasted money. Please seek a second opinion.