> warned that they may have violated the Housing Accountability Act, which says that cities generally can’t deny or downsize housing projects that comply with whatever rules are in place at the time the project application was submitted.
This sounds like a law which states that you can't break the law.
It specifically sets the grandfather date to the project application date - so if I roll into your town and submit a proposal for a building, you can't tomorrow add a "hendrikrassmann law" that requires me to do something additional.
Note that this is NOT the case usually - if you propose a building today and construction starts in 4 years you often have to code-update everything to the construction start date; or even the construction end date. So the law has a purpose.
This sounds like a law which states that you can't break the law.