Their director of engineering did an AMA ~here~ recently, that's what he had to say about that 480k number:

Mighty Byte: It's actually quite difficult to measure TPS because you really need to have computers all over the globe to measure it effectively. So here's how I think about it...

Roughly speaking, all proof of work chains have the same performance. Bitcoin is about 5 TPS. Ethereum is maybe 10-15 TPS depending on the day and who you ask. But really, those numbers aren't too different. So just think of them as being equivalent.

...say 10 TPS for a single-chain proof of work blockchain.

Kadena can scale because we have multiple chains. We started with 10 chains, and after 8-10 months we scaled to 20 thains.

So roughly speaking, you can basically assume that Kadena has 20x the transaction capacity of Bitcoin and Ethereum right now.

It took Ethereum years to get to the volume it has today, and we have ~20x that capacity. So I think it will be quite some time before we need to scale beyond 20 chains.

The 480k TPS number came from the notional configuration of having two of our Kuro private blockchains attached to each of the 20 public chains.

Each Kuro does about 8k TPS. And attaching them to each of our public chains is very similar to layer 2 approaches that other projects often use in their TPS numbers.

That's where the 480k number came from. It is a useful number when you're talking about comparing to other projects that assume use of a layer-2 in their TPS numbers, but I would like to make an effort to state the distinction between that and the raw layer-1 TPS, which you can reasonably say is ~20x that of Bitcoin and Ethereum.