The phrase "multiple tasks at a time" is ill-defined, according to Lamport, because whose clock are you trusting.

For lamport concurrent does not mean what it means to us colloquially or informally (like, "meanwhile"). Concurrency in Lamport's formal definition is only about order. If one task is dependent or is affected by another, then the first is ordered after the second one. Otherwise, they are deemed to be "concurrent", even if one happens years later or before.