Translation:This wood is lighter than metal, but the strength is the same.
"The strength (of the wood) is the same (as the strength of the iron)" is a similar sentence to "(they're) equally strong"
The strength is the same directly translates to "kuatnya sama". And "(they are) equally strong" directly means "(mereka) sama-sama kuat/(mereka) sama kuatnya". You may now understand why it jumped a bit here and there, but you can use either "sama kuatnya" or "kuatnya sama" for this; it means the same in this context.
The strength/"kuat" here can refer to both since they're equally matched on the strength, but it is specifically meant for the wood, as it is the main topic of the sentence.