I have not yet tested it, but I don't see why it should be considered a preferred format even if it is using GPU compute to do the conversion. Either way it is sub optimal, and absolutely pointless. Compressing to ETC2 only to convert to uncompressed means you save no space in graphics memory and take an unnecessary hit in visual quality. Re-compressing to another compressed format means you loose even more quality.
Compression that is decoded on the card still provides a ton of benefits. It reduces the size of data transferred across the bus (a massive performance bottleneck). It reduces the size of your distributables.
Your query is basically saying "I have ETC2 assets, how do you want me to send them to you Mr. video card driver?" The driver doesn't know if you're going to be bottlenecked by bus bandwidth or not, so it wouldn't be right to tell you to send the data uncompressed. The driver also doesn't know if you're willing to trade even more loss of quality for performance, so it can't recommend a different compressed format. A new compressed format might even result in larger data sizes, going back to the bus bandwidth issue. The query most definitely cannot assume you still have access to the original art assets and can convert to a different compressed format from the original source, then change all necessary code and recompile to use a new format. So it tells you to use the compressed format you're already using.
What problem are you trying to solve anyways? From what you've said, I can't really tell what help to provide. It's obvious you're upset the query isn't giving you what you wanted, but people here can't really help you figure out how to get the information you want if you aren't even saying what it is you want.
Well, basically what I'd like to do is compress all of my assets using a lossy codec similar to JPEG for distribution. During installation I would like to find out what texture formats are actually supported by the hardware (the ones that will give the best performance and quality) and then do a one time conversion (e.g. It discovers support for ETC2, so it encodes JPEG -> ETC2. If the card doesn't support ETC2, then maybe it discovers support for RGTC and does JPEG -> RGTC instead.) What I don't want to do is JPEG -> ETC2 thinking the GPU supports it natively and then end up having the driver do ETC2 -> RGTC silently. If that is the case then it would be better for me to do JPEG -> RGTC from the start.