Correct. M206 predates workspace coordinate systems (g54, g55, ...). It is not used (by any sane persons) anymore. It hasn't been removed so as not to break really old G-Code.
For one thing, it is EXACTLY equivalent to G10 L20 P2 (except the coordinate offsets are interpreted the "other way", formally as if monotonically negated). So just use that...
Workspace Coordinate Systems (WCS) are very rarely used on 3D printers. They are quite common on CNC, but even there most people don't really understand how they work. Even some fairly experienced CNC operators.
For now, summarize:
M206 is legacy. Avoid.
WCSs are an intermediate to advanced topic on CNC, and very very rare on 3D Print. Avoid until you have a really compelling reason to dive into them.
Also, re-summary to the very original Q: X0 Y0 for Tool 0 via the axis definitions (and nothing else); each other tool (if any) is offset to that.