It's less about concrete facts I know, just my inclination about it for single-time-use email, the value of one vs the other. And I can't vouch for any specific temp email site (as I said I don't even remember ones I used in the past); but if I know I will never have to revisit or don't want to, I'm less likely to remember the info to delete it from one of the constant-account big sites. I prefer the idea of temp accounts then going into the ether than having remnants floating out there with all its associated services (which can often be themselves signed up for with a separate throwaway anyway if one wants). Especially with the biggest intelligence behemoths and monopolies; who knows what cross-site cookies or contemporary IP or timing overlaps or combined telemetrics get associated into collation in the Utah data center file.

But there is also the "you-shaped-hole in the data" problem too that might theoretically benefit from disjointed spam creation in the services if you do use them regularly for other things (again I have no idea, not a thing I bother with), as well as there is theoretically a double edged sword of too-decentralized uses creating more unique inputs. So who knows. It's just easier and for me feels like less things to think or worry about for a 1-and-done if I know I never have to revisit, and there will need to be more scattered hoops to jump through to pull the inputs to string things together. But this is general low level use cases like the person I was replying to. If objective rigorousness of opsec were a gravely serious concern with something someone was doing one should take it offline in general.