That is correct. RMX can’t decide for you what kind of feel you want in your Bold, how the weight should grow – that’s up to you to decide and draw. You can use the Offset Path (or something like that) to just make your strokes thicker and adjust a ton from there, but honestly, I would just take the time and draw a Bold (or rather, Black) from scratch. Make sure that you keep your two masters compatible: same node number, same curve construction, etc. View > Show Master Compatability can be useful when starting out in this. There was a discussion recently in this thread on how to approach making Bolds, maybe you can take something away from that, if that is what you’re looking for.
Please don’t think that you’ve wasted your money on RMX, it’s an incredible tool and I (like many others) use it as an absolutely elementary part of my workflow. But yes, two masters are the minimum required for anything that has to do with interpolation 