So the general idea is that calendar in spooz is driven by the forward interest rates and expected dividends between the two futures expirations. But also, the very same forward interest rates and expected dividends determine the forward price of the S&P options and those things are very liquid and tight much further out. So if you show a market in the ES calendar spread that has some (even minor) theoretical edge, some HFT OMM will be happy to transact with you and cover the forward risk using options with the same expiration.