Nothing against bud but I don't quite trust retailers for sources of information nor do I think a singular observation in my tank or yours can be decreed as the rule of thumb.
Just because it gets stringy for me in high flow, doesn't mean that's the rule. Nor does the lack of effect of par in stringiness in my tank, doesn't mean your observation is incorrect either.
I just am hesitant to say it is a fact because it hasn't been proven yet either way.
My RBTA is currently at 300 par and is crazy stringy. In my old tank, it was at 300 par, and was bubbly as all get out. The difference was flow for me. That was an observation in my tank only though, I wouldn't consider it a rule. If liveaquaria.com is entirely correct, then my RBTA should have fat tentacles right now.