Export album as set of sequentially-named images to a folder

I took thousands of time-lapse photos and imported them into a few albums in Apple Photos app on MacOS. It wasn't until later I realised Photos doesn't store albums literally in an album:folder mapping.

I don't even know if Photos retains the original file-name or not?

I have software that expects photos to be in a single folder, with sequential naming (like how they normally are on a camera), to create a time-lapse video.

How can I get this out of Photos from an album?

If you select all the photos that you want to export, you can go to File -> Export. You can then chose to export the original files or compress versions. In either case, you can choose to have the exported files keep their original file names, or just be pure sequential (1.jpeg, 2.jpeg, 3.jpeg, etc), optionally starting with a prefix of your choice.