I'm not sure what that is supposed to mean. Given it is preceded by this point, which lists 10s RDs:

• The Tanpan 10 is compatible with Shimano M980, M780, M670, and M610 dérailleurs
• 10-speed Shimano mountain derailleurs may be used with only a slight decrease in shifting performance

I think it means that a Tanpan 11 can be used with a 10s MTB derailleur and 11s road shifter with only a slight decrease in shifting performance. But why would it say? When this is the third point?

• Tanpan 10 allows you to use a 10 speed road shifter with a 10 speed mountain derailleur (Ed. note for post - these are listed above in bold).

IMO it's an error. Tanpan 10 with 10s MTB derailleurs shifts great. Tanpan 10 also works with 9/8/7 shifters & actually improves the shift performance since it reduces the cable pull required relative to the lever throw. You're getting 1.2 shifting with a 1.7 lever, essentially. I've used Tanpan 10 with 4600, 5700, and R3000 with M6000 and M610 and they've been great.

I think the Tanpan is foolproof. You can call and order it from REI so if you don't like it it's an easy return.