I don't think it depends on the Left Party alone, you need the Pirates. Right before the Berlin elections, the SPD and the Greens had about 48-50% in polls, which would have been enough for a comfortable majority considering the parties failing to enter, especially considering that the SPD was fairly close to the CDU and thus the overhang mandates would likely have been distributed more evenly.

the CDU will get more direct seats than that again

Let's check the figures! 38% of the vote, with parties entering parliament adding up to 93%, would mean 40.9% of the non-overhang seats in parliament, or 244 seats in a 598-seat parliament. If the CDU/CSU could reproduce its 2009 record of 24 overhang mandates again (unlikely with such high numbers) with no overhangs for anyone else, that would give them 43.1% of seats – a 2.2 percentage point gain, which equates to the same aggregate loss for all others. Meanwhile the majority of 312 seats, if held by a coalition without the CDU/CSU, would be 52.2% of the 598 non-overhang seats and thus 48.5% of the vote. So with the Forsa poll used for the calculation, only an SPD-Greens-Left Party-Pirates coalition could put the CDU in opposition...

_{*Lunatic*, n.
One whose delusions are out of fashion.}