Tri-motor Plaid powertrain has existed since at least the Roadster reveal, which was Nov 2017 (almost 3 yrs ago). Roadrunner batteries almost certainly did not exist yet back then.
The Plaid prototype demonstrated at the Nürburgring in Sep 2019 was limited to 500 kW motor power. Since we now know the current Plaid prototype has 820 kW (1,100 hp), the 2019 prototype Plaid S more likely had power limitations from the old battery pack, the 18650 100 kWh pack.
We also know that the Summer 2019 Plaid S spotted at the Nürburgring was a returned Customer car repurposed for the development exercise. So less and less likely to have the cast aluminum structural bty tray that was highlighted at bty day. IMHO, this is the primary reason for the delay.
Last year, Elon tweeted that 'Plaid was getting too complicated' and they would go with a simplified version for production in Fall 2020. That will not happen. The new tech must be too good to ignore, too much of a step change not to have it for Plaid. Finally, with Lucid coming too market eventually, Tesla needed to step up with a tad more 'oomph' for Plaid.
Imma say 1.9 sec 0-60 mph, 8.9 sec qtr mi, and 200+ mph top speed gets that done. Ooh, did I mention 520 miles rge? Or 7 seats?
Cheers!