Yes, within VSE (and elsewhere in Blender) almost (*) everything uses premultiplied alpha, directly or indirectly. 8bit/channel images are stored non-premultiplied, but any math done on them does premultiplication, the math, then reverses back into un-premultiplied form (reason being, that w/ premultiplication 8bit/channel is not enough precision). Float/channel images are always premultiplied.

(*) I have found one or two VSE effects that do not do the correct premultiplication within their calculations. Someone(tm) should fix them at some point. But also a good question, whether there should be an option to keep previous "broken" behavior.

wrt color spaces, VSE by default does not do blending in linear color space. Default is sRGB, and variuous blending operations are done directly on sRGB values, i.e. "what four decades of photoshop has taught us to accept as expected results". VSE can be set to operate in some other color space, optionally.

wrt filtering of images, the pixel values are "just" filtered without color space awareness. So for 8bit/channel images (usually sRGB), they get "slightly incorrectly" filtered. If you want proper linear space filtering, can force images to be to floating point (any floating point images wihtin Blender are in "scene linear").

At least the image scaling does not appear to be in linear? Going by the notes in [0]:

    Gimp and stb_image_resize results are subtly different from others, even for Bilinear. Looks like they do convert source sRGB image to linear, do filtering there, and convert back to sRGB.

You can definitely see that as a problem with bright lines (e.g. collar highlight in einar*.png) being darker in the upscaled Blender versions.

[0]: https://aras-p.info/img/misc/upsample_filter_comp_2024/

I’m sure patches are welcome :)