Which is “hotter”? That depends on a lot of factors.
The specific heat of nickel is 0.444 j/g K, while the specific heat of volcanic basalt is 0.84 j/g K. This means that lava requires almost twice the thermal energy to raise it’s temperature by a single degree Kelvin, and consequently if you have equal masses of both nickel and basalt at the same temperature, the basalt will have almost twice the total thermal energy.
That said, in typical usage “hotter” often refers to straight temperature, rather than actual kinetic energy, so…
The melting point of nickel is 1,455°C, but “white hot” nickel isn’t quite molten. The best numbers I could find in a short span of time suggest that many common metals glow white at around 1,200°C, so let’s work with that.
Lava is a little harder to measure. I used basalt above when dealing with specific heat, but lava is actually comprised of multiple kinds of stones, all with different melting points and slightly different specific heats. However, both granite and basalt, two of the most common types of stone you’ll find in lava, have an upper limit on their melting temperatures of around 1,260°C.
So in terms of straight temperature? Nickel has to be at least around 1,200°C to be “white hot”, but it can also be anywhere up to 1,455°C without melting. In contrast, granite and basalt both are definitely molten past 1,260°C, but how much hotter depends on individual circumstances of the lava in question. In the context of this video, the lava is clearly just on the verge of solidifying, so at best it’s around 1,260°C, but at worst it could be as low as about 1,000°C, from the numbers I found.
