That IS what I am saying. Heavy Weapons Squad, Infantry Squad, Conscript Squad, etc, CAN be taken as an individual Troops Choice. They have their own individual Army List Entry. There is nothing preventing it.
There are requirements for creating an Infantry Platoon unit which has requirements and limits, but those requirements and limits are only if you are planning on creating an Infantry Platoon. So, just because they are required for an Infantry Platoon, does not make the Infantry Platoon the only way they can be purchased.
If you take an Infantry Squad as a Troops Choice, you loose out on all the benefits of taking an Infantry Platoon, such as Combined Squad (limited to Squads of the same Choice) and numerous units per Choice, but it is still an option.
Not quite. It looked like you were stating the only way to gain access to an Infantry Squad in a Role Choice is via the Infantry Platoon, "You're right that when building a list normally, you need to build the Platoon as-per the Platoon structure...", implying that the Infantry Platoon is the only way units like the Infantry Squad could be taken outside of a Formation's requirements. This is not a fact and there is nothing written to support this concept.