With the right technology, could the entire blockchain be rewritten?
In theory yes but the alien super computer would have to have hash power at an incredibly large multiple of the hash power pointed towards the Bitcoin blockchain today. According to blockchain.com today's network hash rate is around 130 million terahashes per second. If the alien super computer had more than this it could likely produce the next block with greater than 50 percent chance. However, to generate a new chain from genesis (over 10 years ago) it would need to create 10 years of blocks rather than just one. There are checkpoints in Bitcoin Core at historical intervals for when you are doing an initial block download but my understanding of these is that they don't overrule a new chain with a heavier accumulated proof of work even if it goes all the way back to genesis.