a) The company has the e-mail addresses of everyone who's ever bought soylent, so it's trivial for them to let people know they might have gotten a contaminated batch (or what to look for in case they did).
b) After the mould issue with 2.0 was discovered and acknowledged, they kept selling contaminated batches. One person on Reddit posted about how they had ordered their first batch of Soylent 2.0, then went on the blog and discovered that they'd acknowledged an issue where bottles could end up with mould on the inside. When their shipment arrived, they discovered a majority of the bottles were contaminated.
The problem isn't that there are issues with production, but that the company seems to have no interest whatsoever in being proactive about these issues and preventing things from getting to consumers. What the company should have done is to notify people who'd been shipped bottles of Soylent and cease shipping potentially contaminated products to customers until the issue was resolved. They did neither, putting customers' health (and their brand) at risk.
Maybe GM wouldn't have gotten to the bottom of this immediately, but they sure wouldn't have kept selling products they knew to be contaminated to customers in the meantime.