Def not bullshit.
A lot of the best growers that I follow are now growing this way / starting to grow this way and a lot of them have reported massively increasing their yield and quality. You need one of those moisture meters (teros 12 or growlink) to do it properly though and there’s a lot to it.
There’s a thread on another forum that I’m on where a lot of people who don’t own businesses have switched to it (after buying a meter) and are reporting not just crazy yield increases but quality too.
By having a meter to measure volumetric water content and pore water EC you’re able to fine tune your irrigation practices (with automation) and steer the plant to either produce more leaves during veg or more bud during flower.
As far as the less water content during flower it just means letting it dry out more in between each irrigation and letting the volumetric water content of the media drop and the pore water EC rise to a certain point before giving it another irrigation without going too far (which is not a good thing). Not necessarily giving it less water, hence having one of those meters to tell you when as doing it by feel is not really possible. A lot of fine tuning but as far as I’ve seen once you get it dialed in it’s pretty remarkable.
I know people that are feeding at 3.5-4.5 EC and flowering (aside from last few weeks) at 75% RH and crushing it and most people would have a heart attack seeing those numbers 
The results speak for themselves though. Once my room is more dialed in I’m definitely gonna get a teros 12 and start to learn myself.
Some growers to check out who are using this method
josh_neulinger
jidoka (sacredcowconsulting)
tyler_incognita
craft_farmer
overgr0wn
jenijam16
jonslablab
Probably many more but only so much I can remember.
Edit: there’s a guy currently working on some controller that works with all these moisture meters plus a lot of other things to really fine tune your grows and a few people are testing it and it looks awesome. It’s called pinnacle systems (pinnaclesystemsethoscontroller)