Re-read it again.

The age of the spears, originally assessed as being between 380,000 and 400,000 years old was estimated from their stratigraphic position

However, more recently, thermoluminescence dating […] date the spears to between 337,000 and 300,000 years old, placing them at the end of the interglacial Marine Isotope Stage 9.

All studies place the spears in the Holstein interglacial, which is commonly correlated to either of the mentioned marine isotope stages, MIS 11 (424–374 thousand years ago) and MIS 9 (337–300 thousand years ago).

I’ve edited it for clairty. Let me summarise this.

1. Originally assumed to be 400,000 years old due to location.

2. Later testing shows 300,000 years.

3. All studies so far say that they are either of these two ranges.

Point 3 is a nothing statement because scientists disagree and thus we have leave it open-ended, even though actual testing of the site points to 300,000 years.

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tl;dr: We assumed it was 400,000. We tested and it was actually 300,000. We cant confirm 100%, so we will say it could be either.