It makes no sense. The machine's MO is to shoot a few times then tractor the unfortunate ship into its maw for quick disposal (see the battle with the Enterprise).
How much time did they have to beam an ENTIRE CREW down to a nearby planet IN THE MIDDLE OF A BATTLE? Totally unbelievable. Also a stupid move, considering they knew the machine destroyed planets. Also out of character for Starfleet personnel who would rather "go down fighting" then flee to safety. What happened to Decker's obsession with "preserving life and the safety of Federation planets"? It would have been difficult to stop it without a crew to at least attempt repairs.
Maybe, just maybe, Decker's plan was to get the crew off, hope the planet killer would swallow the Constellation, and detonate the impulse engine inside it. That's the only plausable explanation to me, but why wouldn't he relay the plan to Kirk when the Enterprise comes along immediately? And how could he possibly have time to beam the crew down with the ship already dead (no power, phasers useless)?
Here's what we know of the chain of events as described by Decker:
1) Constellation discovers the machine hovering over the fourth planet, already slicing it up with its anti-proton beam.
2) Constellation engages planet killer - Decker makes the mistake of firing when they are "too far away."
3) At some point the machine clearly devastates the Constellation with its anti-proton beam (evidence of destruction from the planet killer's only offensive weapon is obvious).
4) Constellation is dead (no power, phasers useless) - beam down to third planet commences.
5) After the beam down is complete, the machine "hits again" and the transporter goes out.
6) Planet killer apparently loses interest in the Constellation after knocking out the transporter and goes after the third planet, destroying it.
So basically we have to either believe there is sufficient time to beam the crew down "between shots" or the planet killer is considerate enough to give Decker the time he needs before coming back for a final strike. Why would it do this?
Conclusion: The sequence of events as stated by Decker cannot possibly be correct. The only explanation that fits the facts and the demonstrated behavior of the machine are that Decker beamed his crew down to the third planet WHILE THE SHIP WAS STILL FUNCTIONAL with the intention of using the Constellation for a suicide run, sacrificing only himself and the ship to save the Federation. By traveling to the third planet after the first engagement, Decker escapes the machine's defensive sphere long enough to have time to beam the entire crew down. A long-range communications device is likely beamed down with the crew so that they can contact help after the machine is destroyed. Decker's plans are thwarted by the planet killer's second attack on the ship which fatally cripples the Constellation before it can enter the machine's maw and explode. Decker is then forced to watch/listen as the machine loses interest in the ship and kills his crew by consuming the third planet. The only question remaning is - why lie to Kirk? Why not brief him on the plan to destroy the machine immediately and petition him to help repair the Constellation enough to implement it?
Then again...it IS just a TV show.