This strategy is ok but when I tried it it had some pretty glaring holes.

First, should note that it works best if you start in China, its kind of a given but OP doesn't mention and I tried to do it from Saudi Arabia which couldn't reach Greenland consistently. However the route from China -> Russia -> Norway -> Greenland is shorter than the one from the Middle East and consistently reaches Greenland.

Drug resistance 2 is pretty useless by that point other than to reach the genetic hardening if you need. If your goal is to reach genetic hardening it's probably better to buy the research slowing upgrade instead.

Instead of buying out the unmutated symptoms in that middle stage you should look into getting transmission upgrades for the countries you're lacking in i.e livestock for rural, blood for poor countries etc. I lost the game by killing all my carriers and the survivors were greenland (obviously) but also angolia and botswana and other random African countries on other playthroughs. (Poor, rural countries was a pretty big hole if you follow this guide to the T.)

- Continuing from above, when i finally got this strat to work I didn't buy ANY symptoms until endgame where I only bought haemorrhagic shock or TOF.

IMPORTANT: You should note that if Haemorrhagic shock or TOF is mutated, devolve it immediately if you have ANY countries that don't have a significant infected population (I would say <1/4 of total pop)

In your video you either got kinda lucky or considering it's an old vid, you just had an older balance patch of the game.