> Yes, if there are blocks that are found at almost the same time. But that's not the situation discussed here.

It VERY MUCH is.

Of course if you take another scenario that doesn't make sense, then it doesn't make sense :-).

> They don't know in advance that they will be that lucky.

Whenever you find a block, you know you are one of the first to find it. It's obvious because nobody else has published a block. So you know you are lucky right now. You can decide to wait 1, 2, 5, X seconds before you reveal your block and start mining the new block in the meantime.

Maybe you just mine for 5 seconds before revealing the block, and that's the winning strategy. Maybe you wait until someone else publishes their block and you immediately reveal yours, ending up with two competing chains but knowing that you had a headstart with yours.

The detail of whether or not this is profitable, and how exactly you should do it (Wait X seconds? Wait until someone publishes a block?) is statistics and game theory ("What if the others are also withholding their blocks now? What is their strategy?"). The whole question is whether or not there is a practical, profitable strategy doing that.