What kind of fallacy is this? "Our product is the marker leader, simply because it is better."

One could argue that this is tautology (a pseudo fallacy) if "better" refers to how well the product sells or if "better" is established based on the fact that it is the market leader.

Tautology: Using different words to say the same thing, even if the repetition does not provide clarity. Tautology can also refer to a series of self-reinforcing statements that cannot be disproved because the statements depend on the assumption that they are already correct (a form of begging the question). This is generally not deceptive in argument form.

One could also argue that this is a fair statement if "better" refers to product quality. Some products are market leaders because they have large budgets behind them. Some are market leaders because they were first to market. And some are market leaders for other reasons. So saying that a product is a market leader because it is better than the competition does add information.

Good communication is about clarity. If I were consulting this company making this claim, I would tell them to state HOW the product is better. Perhaps

"Our product is the marker leader, simply because it the quality surpasses that of our competition and users appreciate quality."