I suspect you could argue it from either angle, e.g.:
- Salamander's only able to build his earthquake machine because Gareth improves science's understanding of earthquakes, or
- The Eighth Doctor's intervention means that Gareth's technology immediately undermines Salamander's scheme, changing the future (and hence the Doctor's own past).
Given New Who doesn't show Salamander or any figure like him, it seems highly probable that The Enemy of the World shows a (then) future that no longer exists. The destruction of Skaro in "Remembrance of the Daleks" has similarly been undone by New Who with the Time War, showing the devastated Skaro in "Asylum of the Daleks".