They’re extremely different but there is an element of perceived cross over.
The script lab is in place to help you with the process of building Office JS add-ins but it's not the complete solution. You need to build the add-ins using the SDK's Microsoft provide through an IDE like VS or VS code. It's the cross platform mechanism for building add-ins that work on Excel for web, Windows and Apple platforms.
Office Scripts provides a mechanism for writing Typescript functions that are then able to be executed from PowerAutomate flows.
Also, with Office JS, you can create a fully functioning action pane with HTML/CSS, etc. that your user can interact with. The current extent that Office Scripts provide is a button with script behind. It's really more of an interface mechanism that the user wouldn't typically interact with. They're really powerful when you consider you can mix and match the inputs and outputs with other actions in PowerAutomate.
This is a direct quote from the documentation.
Office Add-ins are cross-platform. They work across Windows desktop,
Mac, iOS, and web platforms and provide the same experience on each.
Any exception to this is noted in the documentation of the individual
API.
Office Scripts are currently only supported by for Excel on the web.
All recording, editing, and script management is done on the web
platform.
While the Office JavaScript APIs for Office Add-ins and the Office
Scripts APIs share some functionality, they are different platforms.
The Office Scripts APIs are an optimized, synchronous subset of the
Excel JavaScript API model. The major difference is usage of the
load/sync paradigm with add-ins. Additionally, add-ins offer APIs for
events and a broader set of functionality outside of Excel, known as
the Common APIs.
An add-in compared to a flow function are very different from a usage perspective and so is the development process. Also, you host your add-in on a web server somewhere when you build it using Office JS whereas with Office Scripts, it's all done for you. The scripts are stored in your OneDrive and the platform has the application model for execution, you don't reference and use that SDK in a self contained project like you do for Office JS.
Some resources to reference ...
https://learn.microsoft.com/en-us/office/dev/scripts/resources/vba-differences
https://learn.microsoft.com/en-us/office/dev/add-ins/overview/explore-with-script-lab