Can em dashes be used to introduce a dependent clause?

Consider the following sentence from WSJ:

And so, like the dutiful and efficient worker I was, I’d put my energy into clearing the decks, cranking through the smaller stuff to get it out of the way—only to discover that doing so took the whole day, that the decks filled up again overnight anyway and that the moment for responding to the New Delhi email never arrived. Source

Following the participial clause (cranking through...), a single em dash is used to introduce another dependent clause that begins with a phrase (only to discover ... the whole day).

Is this a valid use of the Em Dash?

Before addressing the query about the use of the em dash here, it is best to cover the use of only to do something in your particular example.

only to do something: used to show that something is surprising or unexpected:

He spent months negotiating for a pay increase, only to resign from his job soon after he'd received it.

...........

[Cambridge Dictionary]

Coupling the example given above with the following from the Punctuation Guide:

Em dash

The em dash is perhaps the most versatile punctuation mark. Depending on the context, the em dash can take the place of commas, parentheses, or colons⁠....

We see that

• He spent months negotiating for a pay increase—only to resign from his job soon after he'd received it.

is unremarkable. The dash (I'd use a spaced en-dash, a more common, and totally acceptable, variant, at least in the UK) adds a 'pause for reflection' while the comma signifies smoother flow.

• And so, like the dutiful and efficient worker I was, I’d put my energy into clearing the decks, cranking through the smaller stuff to get it out of the way—only to discover that doing so took the whole day, that the decks filled up again overnight anyway and that the moment for responding to the New Delhi email never arrived.

Here, the choice of the colon helps the reader navigate better than addition to the flock of commas would do.