Defining Bitcoin as Bitcoin is a highly liquid asset, even if it is correct, but mentioning it in the wording of ORDER BOOK is incorrect. In ORDER BOOK, the liquidity available on the platform is what may attract traders, meaning that small and medium exchanges have become focused on USDT/low marketcap altcoin paors liquidity. More than its focus on providing Bitcoin liquidity, the reason for this is that many people tend to speculate in these pairs more than in the Bitcoin pair.
Also, the ORDER BOOK differs depending on the platform and trading pair. If it is a platform like Binance, any buy or sell order can be executed due to high liquidity, but in small platforms you may need to wait for the order to be executed.