My understanding is that PowerWall 2.0 either has an AC Inverter or a DC/DC converter.
This is correct. Both versions of the Powerwall 2 have power electronics, which could in theory cost the same to manufacture. The DC connected Powerwall (the one without an inverter) has a DC/DC converter, whereas the AC connected Powerwall has a DC to AC inverter. Both are forms of power electronics.
If you are doing a brand new solar installation, you'd probably go with the DC-connected Powerwall 2. It will be more efficient to use a single inverter for the solar and Powerwall 2 together. No matter how many Powerwall 2s you need, for this type of install, they'll all be the DC-connected version.
If you are installing a Powerwall 2 as a retrofit to an existing solar installation, where you already have a solar inverter, you'd probably pick the AC connected Powerwall, so you don't have the added expense/hassle of taking out and upgrading your existing inverter. Again, no matter how many Powerwall 2s you want for this type of install, they'd all be the AC connected version.
Similarly, if you're doing a Powerwall 2 install without solar (just backup for example) then you'd also go for the AC connected Powerwall 2 to simplify cost and installation.
Inclusion of the inverter not about up-selling as the original poster suggested, it's about compatibility with different types of homes.
Hope that helps.