With 100x TC, one would only need to earn 1500 to make 7 star.
As you said, that promo is definitely for RCs.
I wouldn't be too sure. Tier Credits basically cost them nothing. If 99% of the "prizes" are 3x RCs, then they're laying out 3x the RCs to everybody on 8 days in the month.
$5,000 coin in in a day would earn $30 in RCs instead of $10
But it would earn 3,000 TCs, and then either a 1,000 or a 2,500 TC bonus, so either 4,000 total or 5,500 total instead of 2,000 (1,000 base and 1,000 bonus).
Which would they rather give away? 2,000 - 3,500 extra TCs, or $20 cash? I dunno.
Maybe only one person can win the 100x, in which case, only one person has the potential to make 7 Star on short money.