Heres what I gathered...
Seems in those articles (to me at least) they're alluding to tiedowns but that isn't the case. Tiedowns would pull the structure down (as its name suggests)... but here they used micropiles which aren't tensioned and just basically act like driven piles (like they used on Millenium)... Transbay's foundation system is what the builders for Millenium were aiming for. I'm sure they would have used driven piles at Transbay if it wasn't for the cross bracing for the excavation... noise was probably a concern too. Because of the ability to spread the load out over 3 blocks the ability to use a foundation system that relied on friction to anchor was feasible.
This video was filmed at Transbay in 2012--
• Video Link
In March 2014 newsletter they actually explained the micropiles and how they're constructed. Page 5--
https://tjpa.org/uploads/2014/04/Spr...Newsletter.pdf
More info on micropiles (the 2 pictures at the bottom appear to actually be at Transbay)--
https://drilltechdrilling.com/servic...ns/micropiles/