I must confess I was a bit puzzled when I first read this.
Quickstep and Bora's move to clinchers seems to be their selected choice among the options made available to them by their sponsors/suppliers. I don't have any experience riding tubulars to argue on the clincher vs tubulars, but apparently someone has measured that clinchers are faster.
As road cyclists we have three different wheel tyre systems available - Clinchers, tubular and tubeless. We performed some experiments to determine which has the lowest rolling resistance and is fastest.
www.cyclingweekly.com
What I know from hearsay is that tubulars are more puncture resistant, so I guess that's why teams moving to clinchers may make an exception with the cobbled classics.
I believe powermeters weren't that common back when everyone moved from clinchers to tubulars so they couldn't justify the move with numbers. Maybe if they perceive a rising number of punctures someone goes onto big data to analyse the problem and they move again to tubulars. It's all in the numbers. But what do I know.