As a metallurgist, I'll start with the metallurgist's standard answer, "It Depends." Can you tell us your customer's application? What are the requirements? Do you need cast, forged, or extruded machining stock?
This looks to be a proprietary Al alloy. Here's the Kaiser datasheet for 401-C aluminum:
The chemistry difference is huge, 6061 has a Silicon content of 0.40-0.80 wt% Si, versus 5.5-8.0 wt% Si in the 401-C. This may also impact corrosion properties in the final product. I would agree with the vendor that it would leave silica precipates (low strength, brittle particles) that will function as chip breakers, similar to the effect that Lead has in a free-machining steel.
Compared to 6061-T6 it has slightly higher tensile strength, reduced ductility The comparative properties listed (anodizing response, etc...) don't tell you what the alloy is being compared to.
The data sheet says it can be anodized, and etching prior to anodizing is part of the process.
For the 6061-T6411: A T6511 heat treat condition is Solution Treated and Artificially Aged, along with mechanically stress relieved by stretching ~3%.
Aluminum alloys shouldn't shed particles into moving fluid like cast iron (in cast iron, this would be graphite and very small particle of iron).
When you print the 401-C and 6061 datasheets, the 401-C doesn't have anywhere near the data available that the 6061 does. I'm not saying that the 401-C is bad, just that the vendor needs to provide you the data that would show your customer that 401-C will out perform 6061-T6511 in the final application. Until then, use what your customer has specified.