[random internet remarks]

  20% equity != 20% control

Taking the offer means accepting that the business people don't rate you as someone worth sharing business control. That may or may not mean that you are someone worth sharing business control...i.e. their assessment may be inaccurate. It may be an intolerable blow to your ego. But that's the world described in your question (there might be more to it, but that's all I've got).

If the offer comes with a market rate salary, it smells like an above average deal and probably reflects a board that doesn't want you distracted. It also reflects a board that has adequately capitalized the company for operations (and that's the metric of the "business person" in a company). A market rate salary is plausible compensation for the risk inherent in not having business control. If the salary is below market rate, it smells closer to the "two business 'guys' hire a programmer" end of the developer-as-entrepreneur risk spectrum.

There's nothing amiss. It boils down to whether or not you trust the two partners, your objectives, your current career and economic position, and your assessment of the risk. Good luck.