As someone said - OMNI rides on BTC network.

You'll notice that many Exchanges list ERC20 tokens more readily and easily than entire new blockchain projects - and the ERC20 tokens might all have the same deposit address like you noticed with BTC and USDT.

As you might guess, this is because doing the backend work for an Exchange to list an entire new blockchain is a lot more work.

This can make it tough for the smaller blockchain projects out there that are trying to build a whole new ecosystem to get exchange listings. It is often easier to get listed on smaller exchanges if you have a token riding on Ethereum.

So if you see a small Exchange list an almost unknown blockchain/coin (not ERC20) - it is likely a lot more work!

So in short, it's all good if BTC and USDT are the same address, and equally no issues if ETH and ERC20 tokens have the same address on an Exchange.