This website https://www.thespruce.com/install-floors-over-radiant-heating-systems-4121256 lists the worst floors for installing over radiant heating include "Very Dense Solid Hardwood:  Hardwoods that rank 1,375 or more on the Janka scale are a poor fit for radiant heating systems."

Whereas this website https://www.warmup.co.uk/underfloor-heating/floor-coverings/wood-laminate says "When choosing the wooden or laminate flooring ensure that the material is as dense as possible for good thermal conductivity. The denser the wood, the better it transfers heat."

Huh?  Which is it?

It's an imaginary tempest in an imaginary tea pot.

With a reasonably designed radiant floor system it can deliver the heat with any type of wood, with no risk of damage to the wood.

I'd focus on other attributes (like dimensional stability). Engineered (laminated) wood flooring has good stability and low R-value (for wood).

The higher the R-value above the PEX tube, the less heat that goes upward into the living space and the more heat that goes downward and is lost resulting in a lower efficiency HR emitter. Softwood (lower density wood) tends to have a higher R-value than hardwood (higher density wood). So dense hardwood would be better from a heat conduction and HR emitter efficiency perspective.

As long as you can get enough insulation below the PEX tube, you can deal with the poorer heat conduction of having wood above the PEX tube. A 95% efficient HR emitter is a good target. If you have R1 above the PEX tube, you will require R20 below the PEX tube to achieve 95%. If you have R2 above the PEX tube, you will require R40 below the PEX tube to achieve 95%. The more R-value you have above the PEX tube and the more mass you have surrounding the PEX tube also results in a less responsive HR emitter which can be problematic in a less than well-insulated and well-sealed house that is sensitive to a changing outdoor temperature and needs a responsive heating system.

OK thanks for the replies!