Correct
BOOSTING is not ruled out, however UPGRADING builders huts is ruled out.
And if you consider that a boost would (logically) cost a similar amount to gemming the remaining time at the end of a build, this is the sort of cost you'd be looking at.
Gem ONE builder for ONE day for DOUBLE speed (so one free day) is 260 gems
Gem ONE builder for ONE day for QUADRUPLE speed (so three free days) is 506 gems
http://clashofclans.wikia.com/wiki/Gems
Who's going to pay prices that are ANYTHING like that?