Without going into details, we can make the extracted binary hard to analyze via, e.g., obfuscation or other methods. You are correct, just using a decryption k

Without going into details, we can make the extracted binary hard to analyze via, e.g., obfuscation or other methods. You are correct, just using a decryption key in ROM without other countermeasures does not provide much security as it can also be read with the same method. You need to assume the entire flash image is known and built from there. As you said, traditional code obfuscation is not practical on these small devices, but this is exactly the problem we wanted to tackle!