I didn't see charts showing HDPE resistance, so I can't comment on that.

But, based on my experiences with Chinese plumbing product quality control, my working assumption would be that Chinese-made, Lowes-bought PE tubing *might* be HDPE (which might, or might not, be adequately resistant -- I don't know), or it might be LDPE (which is not sufficiently resistant) or might might be any other plastic that merely looked and felt like LDPE.

Using tubing that "becomes brittle" -- your words -- to transport chlorine gas is definitely on my "To-Don't-Do" list.

And, anyhow, in most of the areas I'm familiar with, chlorine gas use on pools is very actively discouraged (if not outlawed) by current regulations, so it's sort of a non-issue in those areas.