Right now the explorer uses timeFirstSeen for txs that are unconfirmed, i.e. do not have a blockheight. For txs with a blockheight, it uses the block timestamp.
This is perhaps more "right," but it is not as useful as timeFirstSeen.
For example, when I check the token server wallet, it is not useful at all if I see 20 txs with the same timestamp. Maybe they are all in the same block which took an hour to find.
However if I see 20 txs that are all coming a few seconds apart, this is a good sign there is some kind of a script running.
timeFirstSeen imo provides more useful information.