Yes, the block of transactions is arranged in a Merkel tree, but that Merkel tree is finalized, chained to the previous block and secured by proof of work. Lib

Yes, the block of transactions is arranged in a Merkel tree, but that Merkel tree is finalized, chained to the previous block and secured by proof of work. Libra is changing the Merkel tree over time, it is never final, which is similar to pre-Bitcoin digital currencies.