They could be differentiating between Traditional Mandarin, and mainland simplified mandarin which is more commonly accepted as “Chinese”. Especially since traditional mandarin is still the official written form of Chinese in Taiwan, as they rejected the Maoist era reforms.
Either they'd also be happy with e.g. Cantonese or they're hedging their bets and don't know who they will offend by suggesting that Taiwan speaks the same language as the most common one on the mainland.
Are they implying Mandarin is not Chinese?
They could be differentiating between Traditional Mandarin, and mainland simplified mandarin which is more commonly accepted as “Chinese”. Especially since traditional mandarin is still the official written form of Chinese in Taiwan, as they rejected the Maoist era reforms.
Either they'd also be happy with e.g. Cantonese or they're hedging their bets and don't know who they will offend by suggesting that Taiwan speaks the same language as the most common one on the mainland.