Strictly and only addressing the black light test picture, that is a good sign. It appears the poster as shown does not have optical brighteners (comparing it to the foam). If it fluoresced as bright as the foam backing, that would prove it modern.
An antique poster would have no optical brighteners. However, lack of optical brighteners doesn't prove it old, as not all paper modern paper has optical brighteners. The lack of optical brighteners just says that that specific quality is consistent with it being old, not proof that it is old.
Antique items can and usually do have some fluorescence (they almost never show up as black under blacklight). It's just that they shouldn't fluoresce as brightly as a modern piece of computer paper or baseball card. That's why you should do the test by comparing it to something modern that fluoresces brightly, such as the foam backing. The poster's fluorescence is clearly quite a bit darker than the foam's. I often use a little shard of computer paper for comparison. Something with optical brighteners is usually obvious, as it very brightly.
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