That said, the ADA is all about accommodating disabled persons within reason. Depending on the scope and functionality of his software, fully supporting such with a single developer on staff MIGHT be an undue burden - it all depends on the specifics of the situation and we don't know his.

Notice the usage of "Undue Burden" from the official ADA documentation below.

FROM: https://www.ada.gov/regs2010/smallbusiness/smallbusprimer201...

"The rules are also flexible for communicating effectively with customers who are blind or have low vision. For example, a restaurant can put its menu on an audio cassette or a waiter can read it to a patron. A sales clerk can find items and read their labels. In more complex transactions where a significant amount of printed information is involved, providing alternate formats will be necessary, unless doing so is an undue burden."

"It is a business's responsibility to provide a sign language, oral interpreter, or VRI service unless doing so in a particular situation would result in an undue burden, which means significant difficulty or expense. A business's overall resources determine (rather than a comparison to the fees paid by the customer needing the interpreter) what constitutes an undue burden."

It's also very possibly their developer is woefully overworked, and what may amount to 'minimal effort' for you, could be outside the scope of what they have the time to accomplish.

Come on now. Stereotyping is an affirmative action, failing to consider accessibility is passive / apathy. It's comparable to not installing a handicap ramp for your restaurant.