This is not true. You mean you have no default risk because it is issued by the US government. Everything has market risk. Next month you will be able to buy a 10 yr UST bond at 5%. If the market expects 3% inflation your 1% real TIP is worth less if you have to sell it. If you don't sell and the real yield on the bond is more than your TIP then you have lost money in opportunity cost and have less money than you could have. You also paid tax on income not received (if taxable account) and did not reinvest the coupons at a higher rate. It is math, interest rate up = price down for all bonds. Not all bonds lose value in the same amount (depends on duration and cash flow) and some bonds have credit risk in addition to market risk.
These issues are magnified by bond funds because they have expenses and cannot 100% control when they buy and sell. It does not matter how good they are at picking bonds if their monthly cash flow is negative, they have to sell to meet redemptions. Bond funds have more market risk than individual bonds which is why we are recommending to hold the bonds directly. If you are afraid of credit risk buy Treasuries, if you are willing to take some credit risk for the extra spread then due your diligence, diversify, and stay out of industries that have historically had a lot of defaults due to the business cycle.