Yes, bonds for reserves, and vice versa. But recall that reserves are money -- they are the key component of all measures of money. Also, recall that the primary dealers which trade with the Fed routinely act as intermediaries on behalf of third parties -- mutual funds, companies, individuals, etc. During QE, the Fed was buying bonds previously held by the private sector. Now, with QT, the Fed is letting the treasury/agency bonds it holds mature (it may be selling some of those bonds too), canceling the money it receives, and thus putting the private sector in the position of having to buy the new treasuries/agency bonds that refinance the recently matured ones. So, I think it's OK to simplify things as the parent comment did for the purpose of having a useful mental model.
Adding to this, the key is what these market actors purchase in place of the bonds that were sold.
They shift out and buy more risky assets. Corporate bonds, equities.
And then next the people that held the corps/equities that were sold buy still more risky assets (e.g., speculative equities, VC) and so on (e.g., crypto).
