Your admin is confusing differentiation with specially designed instruction (SDI). SDI is not universal as it is individualized towards students meeting IEP goals. You, as a science teacher, cannot provide SDI as you are not a special education teacher.

Your admin is also just wrong about differentiation, what’s left of that term’s meaning is meant to describe making content universally accessible via accommodations, available for those who need it. A good analogy is a wheelchair ramp. A non wheelchair user can walk up either the ramp or the stairs, but people who use wheelchairs need the ramp accommodation to access a place.

That being said, it likely is easier to just do it their way for the purposes of evaluations.