talking about this oneL: https://www.bulkreefsupply.com/36-hybrid-t5ho-4x39w-fixture-aquatic-life.htmlThey announced the DX18 series mid last year, it is dimmable. The older model was not. BRS has both on their website.
Here is a non-dimmable 36" version on sale: https://www.bulkreefsupply.com/36-hybrid-t5ho-4x39w-fixture-with-led-mounting-system-aquatic-life.html
Here is the dimmable DX18 36", not on sale: https://www.bulkreefsupply.com/36-dx18-hybrid-dimmable-t5-ho-light-fixture-aquatic-life.html
It clearly says dimmable. Maybe you are seeing something different.