Kopiko's tagline here in the Philippines became a hot topic. Others had been saying that it is grammatically wrong but others said that it is correct.
There is an implied notion of time in the statement. Until now or recently this was our creamiest but our changes to it have made it creamier now. As a similar example, our tallest (until recently) daughter is even taller (today).
The key to understanding this grammatically is that in the superlative of yesterday is compared with the superlative of today. Hence, today’s best (superlative) is better (comparative) than yesterday’s best.
British Council : comparatives and superlatives.
I think Kopiko have lessen their cream content that's why from creamiest it was demoted to creamier. Less cream more coffee. But they shouldn't go further to creamiest instead they should've created new variant of Kopiko. Because just like Best now it's Better. Best is not always the best.