It wasn't uncommon for flak guns of all calibers to be pressed into use as anti-tank weapons on land in an emergency. Some like the vaunted German 88mm AA gun proved superior to most weapons designed solely for that purpose. That being said, I would think punching through the comparatively thin steel of a small freighter should be no problem for a 20mm. They won't be big holes, but they'll take their slow toll. It'd certainly keep the crew's heads down and keep them from making repairs.
As for the big ka-boom at the end...I can't say.
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