Poorly applied crypto is far more common than weak crypto.
We see many mistakes: No IV, ECB mode, keeping weak version along with strong, weak keys, error correction underneath encryption and many more.
In this case they didn't actually apply encryption to the entire feed only rotated the lines pseduo randomly, and the decryption did not require breaking the key nor even figure out which encryption algorithm was used.
As for how good the military is in using it? well The Israelis got caught with their pants down, the US had their totally unencrypted feeds captured by Taliban in Afghanistan, and had a drone hijacked in Iran.
So If two of the most technologically advanced militaries are obviously making embarrassing mistakes, do you think any military is safe?