it's definitely pay to win, you can say it as you like "invest to win" or anything else. I have been playing card games since i was a kid, so i have at least some experience in this matter.
Let's take 2 of the most online popular games today. It's duel links which is yu gi oh and heartstone. Heartstone when it started it was 100% pay to win cause you had some basic cards for free, you earned other basic cards for free but if you paid money to buy packs you could win the best ones. Thus those that paid 0 $ had literally no chance of advancing in this game.
Now because they added many cards and different ways to earn cards for free (by grinding if you have heard the term) even if you pay nothing you can advance pretty far.
Duel Links on the other hand made it pretty easy to earn cards. For example i haven't paid nothing and have let's say for example 80% of the cards. You need to pay quite some the amount to get the best cards and there is no point. The way they did it though, doesn't require any skill as anyone can find the cards, check for decks online and build a pretty pretty decent deck. For example i said to a friend that had no idea about yu gi oh to start the game and buy only a specific pack multiple times (with in game currency not real money). In less than a month he mad a deck that led him to the second best division :P
i think steemmonsters is definitely a pay to win game as you literally only pay to gain packs. Sure there are some giveaways but the main way to gain packs is through buying, except if you are all day in the contest tag and look for free cards. Thus it simple stats, in order to gain more cards you need to buy more and if you buy more you will obtain better cards and build a better deck.