Is this article wrong/scammy? Is it possible that miner earn more by mining several different coins at once through minergate?
You can possible optimize revenue by switching to mining another coin dynamically, but it isn’t possible to mine two different coins in a given grind attempt.

The exception are chains which are merge-mined. Each merge-mined chain block is committed to a block of a main-chain block, a Bitcoin block for example. Validators of the merge-mined coin can validate POW of the main-chain (eg Bitcoin) and enforce separate consensus rules.

The difficulty of a merge-mined coin can be equal or higher than that of the main chain.

Therefore, if there is any significant value in the merge mined coin, a Bitcoin miner would be incentivized to include valid block commitments in each Bitcoin block it attempts to mine.