Monero

I know Ruckinum ran an analysis and thinks this is not a black marble flood, but I can’t help but think it’s a way go somehow break the anonymity of monero, whether just sent amounts, or received amounts, which would still give a wealth of information.

I didn't run a quantitative analysis of the large number of 150-input transactions on May 2. I just guessed that it's not an actual black marble flood since it doesn't fit the definition or attack model of Noether, Noether, & Mackenzie (2014) and Chervinski, Kreutz, & Yu (2021).

Are the transactions reused?

Yes, each output can be re-used an unlimited number of times as a decoy in other transactions.

If they are reused, then they can tell the real spend by discarding any spend that’s been used more than once. Is that correct?

No. If every output that is created is spent, then on average each output will appear in 16 rings of other transactions. A Monero wallet do not check how many times an output has been used by other transactions when it is deciding which outputs to select as decoys.

They run or have compromised a lot of ‘activist’ nodes and xpubs are sent to the nodes in light wallets, unsure if this is how it works, or if that was unique to Samourai’s whirlpool design. If this was the case, light wallets use currently online available servers, so chances are a user connects their wallet to tens of servers. Users who run their own nodes would be unaffected but I think the majority of monero users use light nodes.

In normal operation, most Monero wallets do not send an "xpub" (in Monero this would be the Private View Key). The terminology can be confusing. In Monero, a "light wallet" is a wallet where the user gives a view key to a server to perform the blockchain scan on behalf of the user. The person or company running the server can see which transactions belong to the user and how much XMR is being sent to them. The MyMonero wallet works like this. Feather is not a light wallet with this definition, despite its name. Feather wallet and most wallets like Cake, Stack, the GUI/CLI wallets, etc., ask a local node (on the user's own machine) or remote node (on someone else's machine) for the entire blockchain data during a period of time and do the decryption of the wallets' transactions on the user's own device. That's why wallet sync takes a long time for those wallets when they are opened after being closed for a long time.

The remote nodes can collect some limited data like the user's IP address (if the user is not using Tor) and the last time the user synced the wallet. A malicious remote node can attempt to give the user a false decoy/output distribution (this is what Feather was trying to prevent with the initial, but flawed, code) and it can give the user's wallet an incorrect fee to pay (but the user can notice that the fee is too high and disconnect from the remote node. More information about remote node privacy is in Breaking Monero Episode 07: Remote Nodes (sorry for YouTube link. Use your favorite private YouTube front-end to view it): https://www.youtube.com/watch?v=n6Bxp0k7Uqg