How are optimistic rollups limited by their parent chain?

In a recent AMA with the ZkSync team, Alex, one of the cofounders, mentioned that--

"No matter how you optimise their data compression, it will remain linear in terms of number of transactions, and the throughput of all ORs combined will remain strictly limited because all rollups compete for the same limited ETH block space. Transaction costs for all rollups will keep growing.

ZK tech, on the other hand, enables truly infinite scalability via validiums. They come with certain security tradeoffs (and we spend a lot of effort on mitigating them with strong decentralisation), but this is the only technology we have today that can create a practical Internet of Value for millions of parallel users."

Could someone ELI5 this? Is this saying that optimistic rollups are limited because they have to compress tx details in call data? And because there is limited space in call data, and limited/expensive space in blocks, you can't scale efficiently?

Link to the AMA: https://www.reddit.com/r/CryptoCurrency/comments/u1dyqj/comment/i4ffz0t/