The given translation is a good one.
Given some scale, だけ
either excludes the lower side or the higher side of the standard. Probably you only knew the usage of excluding the higher side. That usage is more common.
- Excluding the higher side of a scale: 'no more than', 'nothing other than (= only)'
- 寿司だけを食べた 'I ate {nothing other than/only/ at most} sushi'
- Excluding the lower side of a scale: 'no less than', 'as much as'
- 日本に行っただけのことはあった 'It (= the outcome) was worth {no less than/as much as} (the cost/effort/etc. of) going to Japan'