Am I missing something here?
With very few and rare exceptions any unit in the game may choose to start the game in reserve. Broadsides certainly may do so.
Rather than missing something I feel you may be reading too much into this - if you simply follow the instructions in the Low Altitude Deployment rule as written you will have no problems.
The Broadsides do not directly gain the Deep Strike special rule in the Retaliation Cadre, so they could not for example be placed in Deep Strike Reserve and arrive by Deep Strike normally after rolling reserves. They do gain the Low Altitude Deployment rule and subject to its restrictions arrive using the Deep Strike special rules - the restrictions are that all models in the formation must use the rule or none may and that they arrive on Turn 2 regardless.
There are plenty of places in the game where a model that does not itself have Deep Strike ends up using the Deep Strike special rules, it is a mechanic that the game designers like to re-use.