My reading is that it isnt just permission to claim all development as R&D, but an obligation to do so if you claim it all if you claim any. That is to say, no splitting it.

This is actually pretty common in other parts of the tax code, where you have discretion in how to treat costs, but different treatments come with different requirements. Selecting one treatment is what triggers the associated requirements.

Many people struggle to understand that the tax code is not just a set of rules, but also a set of choices.

As an aside, this is why the government cant simply calculate your taxes for you in many instances. Doing so involves several choices based on your long term strategy. A simple example is to take losses in the given tax year or carry them forward (when permitted). The IRS can't decide that for you, and your decision might depend on factors like if you think your tax rate will be higher or lower in the future.

Perhaps a more relatable example is when to take a IRA/401K withdrawal. Ideally you would want to do this in years in a year where you have a lower marginal tax rate.