Trading only Bitcoin will reduced the risks that you will be exposed to in the market because the market is already very volatile and when you add other unstable cryptocurrency, it might become a problem to you when your analysis gives you bad results not because you did anything wrong but because the altcoins that you are trading is having some challenges that you did not know about. Altcoins can be easily manipulated as some people have too many large amounts of those altcoins with them sitting in their wallet that they can use to manipulate the market as they like. But trading only Bitcoin means you would not be exposed to those market manipulation by the whales or others with the power to make the market go their way.
I agree with you that trading only bitcoin will significantly reduce the risk compared to trading altcoins but it would be wrong to say that bitcoin is not manipulated and we will not face those risks.
Admittedly, Bitcoin is the coin with the largest market cap and manipulating it may be harder than altcoins but that doesn't mean Bitcoin is immune to manipulation. Cryptocurrency is a market that is still quite free and unregulated, rife with manipulation and Bitcoin is no exception. Market volatility is largely due to manipulation, so as long as bitcoin remains highly volatile it means bitcoin is being manipulated.
The only way we can avoid market manipulation is to invest and hold for the long term.