Hi, I recently came across this fact: The largest root system ever recorded was on Finnish Pine tree with a total root length of 50km and over 5 million tips!

How true is it? Does this talk about depth or spread of roots?

I would think the spread of roots appears to be what is visible above ground. Not having seen this, I can only confirm what you have found, but if you wouldn't mind please post a link to your information. Not having any arborists here to give a qualified answer is rather unfortunate.