An instant crypto exchange is an exchange solution that aggregates prices and liquidity from multiple custodial trading exchanges. Exchanges like Binance, kucoin and bittrex are called custodian exchanges.
Cryptocurrencies and other funds like US dollar are safe or safer on instant exchanges because they are save on your wallets. You retain the full owner of your cryptocurrencies because they are save directly on your wallet in which you have your private key and seed recovery phrase. So, you have more control than on custodial/traditional exchanges. Exchanges can be hacked not like wallets that have private keys features.
But know that if you want to trade, you need to utilized custodian exchanges which means your funds are custodied at this time.
Instant exchanges registration is very easy and you can sign up fast with just email address and a user name. It is more modern and easy to use.
But, the trading fees are higher than custodian exchanges. And not easy to use by novice that have little knowledge of how to protect crypto wallets.
Exchanges using the same wallet where you are keeping your funds are rather instant , they have a fixed price about buying and selling of bitcoins and it does not cause you a great deal of loss , you actually pay less that what you were gonna pay when you are transferring your BTC from one place to another.
This is the only kind of instant exchange I have seen , the one that you stated kinda seem risky , they might be into scamming you or something , the security does seem very bad , I do not trust anything which is online at all.
Now the wallets itself have this integrated options where you can trade according to your preference I think that is a safer option , keeping your money online is never right.