The trouble with this method is that whales do not announce they will buy something before they buy it, hence you can't really know anything beforehand. It's going to be pretty hard to determine what the yare going to buy and that is why we should not be expecting that well right now, it's just not that easy. Of course it's not impossible, some people do it, but you do not have 100% correct rate anyway.
This is no longer a problem again, there are bots now that you can use to track an address on some of this bots you can’t have a script that it should buy anything that the address is receiving, you can even set a price in usdt that it will use to buy the token. This way once the address gets to buy and receive any token the bot also buys for the person and this actually makes one early.
This is very common to Degen traders Who usually trade meme coins, they simply track whales and some insiders to copy there trade, this usually makes this traders early but the problem is that most of this whales have multiple wallets and some of the wallets this traders are tracking are usually not even their main wallet