I thinks these are the features which separates S series to S Lite series. To protect their flagship they have to do it. Though it has the capability but if they start supporting DeX then it may increase the cost of development.
Your point is acceptable.. But these two lite phones are not cheap. But we dont have Gorilla glass 5 protection, glass build, ip rating, 90hz refresh rate. For 43000 INR they can give just Dex support. Expect this in next software update.
i think its the difference in usb port gen. on a90 it is usb type c 3.0 (and its is also 5g) so it supports Dex just like flagship s and note series. it is merely a speculation but i think Dex requirs usb 3.0, that's why it is not supported in s10lite as well as note 10lite as they both have usc type c 2.0.