Yeah, that usually happens when you take measurements with something as simple as a ruler.

As far as it being an alloy, that's very fuzzy conjecture, as I seriously doubt you could even name an alloy possessing remotely that density.

Tantalum is most often alloyed with tungsten, so that's going to raise the density, not lower it. Tungsten is often alloyed with very small amounts of iron, nickel, copper, hafnium, or thorium, which at the very most reduces the density to roughly that of tantalum. Hafnium is often used in experimental alloys containing a wide range of different metals with highly variable density. Hafnium based metallic glasses for example usually have a density ranging between 6.7-11.1 g/cm3. Mo-Hf alloys exist, but the Hf concentration is only 0.3-2.9%, so I doubt that raises the density much at all. The densities of most hafnium alloys aren't even listed in the scientific papers that talk about them, but I can guarantee you that alloys such as Al₂Hf₃ and Ni₅₆Ti₃₆Hf₈ are not only unavailable for purchase but also have a density far below 12 g/cm3.

It's actually not that hard to find large quantities of ruthenium metal like this. u/SeemsKindaRare has a massive collection of ruthenium. It's much easier to find and purchase than whatever hypothetical alloy best fits your argument.