Good question, tbh, those numbers are from memory from my geology classes back when I studied Geo Science. I may have remembered wrong or those numbers where wrong back then.

Edit: searching a bit more I found this (unverified) answer that might explain the huge difference:

>As the present temperature of the inner core is estimated to be around 5000∘C, this is going to take tens of billions of years.

>The magnetic fields are generated by eddy currents in the outer core, which is a liquid layer about 2,300 km thickness. The inner core is growing at the rate of about 1 mm per year, so it is going to 'freeze over' (i.e. solidify) in about 2.3 billion years. Without its liquid outer core, the Earth's magnetic field shuts down,

https://earthscience.stackexchange.com/questions/7036/how-long-would-it-take-for-earths-core-to-cool-down-and-solidify

If this is correct, the whole process of cooling down will take ~90 Billion years, but the magnetic field will collapse much earlier, making complex life on earth impossible (which will be the case anyway due to increased sun luminosity, but that's another question.)