The warning is telling you that you are using a corridor feature line instead of a toolkit corridor feature line.
Basically, when you get objects into dynamo, you get a proxy of your C3D objects. So in dynamo these are 2 different objects but they both come from the corridor feature line in Civil 3D. If you see the first screenshot I shared, You can get the TkFeatureLines from the corridor and then work with this object to get the points.
The reason TKFeatureLine exist is because there is a problem with the geometry representation in Dynamo, see here : CorridorFeatureLine.PolyCurve jumping gaps - #9 by marshmallow