In simple words, market cap is the total value of a certain project at this moment.
That's not accurate. The market cap is simply the most recent price multiplied by the circulating volume. It doesn't say very much about the total value whatsoever.
Example: I create a new token with a supply of 1 billion. My token has no function and serves no purpose. Nobody wants it. I go to the altcoin boards and shill it, and some newbies fall for it and want to buy some. I find someone to buy 10 tokens for $10. The price of each token is therefore now $1, and my token now has a marketcap of $1 billion and has immediately become a top altcoin. Does my token with no purpose and no function suddenly have a "total value" of $1 billion? Of course not. Marketcap is next to useless when the supply can be artificial created and managed by the creators of the project.
As BrewMaster said on the post you linked to, many altcoins realize this, and if they released all 1 billion tokens at once they would make no money whatsoever (which, let's be honest, is the only reason the vast majority of altcoins even exist - to make their creators easy money and rip off "investors"). So they release 1 million tokens, sell them for $1, still end up with a ridiculously over-valued marketcap, and then can slowly sell the 900 million extra they were keeping themselves for a profit.
So what could represent the value of a project if the market cap can be manipulated
There is no single good metric to use. Volume can give an idea, as in my example above, if you saw a project with a marketcap of $1 billion but a volume of only $10, you would know it is a scam. However, we also know that many of the top scams such as BSV pump out loads of fake volume, exactly to trick people in to thinking it is popular when it isn't.
what the purpose of using billion of token as supply
Many newbies don't pay attention to the supply, only the price. We regularly see newbies buying scam coins like XRP because "they are much cheaper than bitcoin", not understanding that there are 100 billion XRP in existence and so the price will never be more than a dollar or two. Not to mention the creators are "holding" over 50 billion which they dump a portion of on the market whenever they want some profit.