Yes, it would technically meet the Open Source Initiative's definition (https://opensource.org/osd), but if there was no way to re-flash the device, no way to verify the binary on the device, or possibly even no way build a binary (which may require proprietary tools under NDA from the chip manufacturer) -- I think a lot of critics would still be critics, but I could be wrong.

If Yubico did this it would be very interesting to see the reaction.