In that transaction I see a "sender 0", which would be Coinbase, and 721 recipients.
Are the 721 transactions
1) those that miners validate in that block? or
2) is the miner sending the reward to 721 different addresses?
Option 2 as explained by pooya.
Just to expand on what you suggested for option 1 - the outputs in a coinbase transaction have nothing to do with the other transactions which are included in that block. The majority of coinbase transactions now include a single output containing the block reward and all the fees they are claiming, plus a few additional OP_RETURN outputs, usually marking the name of the mining pool which found the block. If the miner chooses to split the subsidy between multiple addresses (even between several hundred addresses as we've seen here) instead of just sending it to a single one, then they are free to do so. This has no bearing on the other transactions included in the block. A block with 1000 transactions could send the whole block reward to a single address, and an empty block with no other transactions could split the block reward across 1000 addresses.
