This is not usually the purpose of sharding though. Having a replica of each node or each block of data (and a good failover system) is what would allow you to pull a node offline with no impact. Though it's worth pointing out, even if they had a replica of the node in this case, the replica would probably experience XID wraparound at the same time so that probably wouldn't help.
Sharding usually means partitioning the data so that different data goes to different nodes. In this case that's consistent with 20% of outbound emails being affected if 1 of the 5 shards is down.
There are definitely some red flags with their usage though, like ideally only 20% of inbound emails and events would be affected as well but they said almost all of them are. And ideally you couldn't get into a situation to begin with where you have one shard getting way more writes than everything else. And of course ideally you're monitoring XIDs and can respond enough in advance. I'd be interested to read a more detailed writeup, though based on some of the comments here about their lack of transparency it seems unlikely that one will be released.