It is junk, but "space junk" has a very specific meaning [1] and this is not it. Pretty much all rocket launches have debris designed to fall back down into ocean, some recovered and some not. Not only that, but defunct satellites are intentionally crashed into the ocean and left there [2].

There is literally nothing special about this part other than perhaps it floated and ended up in another country.

[1] https://en.wikipedia.org/wiki/Space_debris

[2] https://en.wikipedia.org/wiki/Spacecraft_cemetery

Much like how "garbage collection" is industry terminology referring to a specific feature of memory management in programming languages, "space junk" is also industry terminology that's specific in its meaning:

> Space debris (also known as space junk, space pollution, space waste, space trash, space garbage, or cosmic debris) are defunct human-made objects in space – principally in Earth orbit – which no longer serve a useful function.

Source: https://en.wikipedia.org/wiki/Space_debris

If it's no longer in space, it's no longer "space junk".