It would be a mistake/inconsistency rather than a retcon.

But it isn't really. The MCU has multiple different explanations for how time travel works through different methods.

This is one of them.

It's a bit difficult to explain logically but it is what it is.

It’s different forms of time travel, quantum realm, Monolith, chase stein’s device. Nobodies sure if they should be the same or can be different

Runnaways time travel was inspired by relativity, rather than quantum physics. Nothing wrong with that.