That might be what some weaselly lawyer argues it means in court, or what someone deeply familiar with it all would infer. But in no way is that how any reasonable lay person would read it.
"Improve functionality over time" implies that the basic functionality of full self-driving is there already, and that it will be improved. No reasonable person would take it to mean "does not exist yet, and may never do".
They would also take "capable of" to mean "right now", not at some nebulous point in the future, especially given the context of the feature being a paid upgrade.
The definition of "full" is "not lacking or omitting anything; complete". One might reasonably expect "full self-driving" to mean a human does not need to intervene or monitor. I.e. What the industry commonly calls autonomous driving level 5. They clarify what they mean elsewhere on their web site:
> Autopilot, Enhanced Autopilot and Full Self-Driving Capability
> are intended for use with a fully attentive driver, who has their
> hands on the wheel and is prepared to take over at any moment.
> While these features are designed to become more capable over time,
> the currently enabled features do not make the vehicle autonomous.
So they don't even claim level 1 autonomy.
Even with your version, the hardware may very well never be capable of full self-driving at all. There's no evidence to suggest it will be - hopes, dreams and promises don't count. And they've done an upgrade on the earlier-gen Model S cars already, where they presumably made similar claims.
The whole thing is misleading at best, downright trading standards fraud at worst.
For what it's worth, my Model 3's carriageway departure warning kicks in randomly and the amount of phantom braking it does even on basic cruise control is terrible. Autopilot is worse. I find monitoring it in autopilot mode significantly more stressful than just driving the car myself. I wouldn't trust FSD at all, and certainly not enough to want to fork out the silly money it is.