This argument is a weak one. It is very unlikely that members of Congress often feel pressured to act as trustees rather than as delegates.
In the trustee style of representation, the legislator acts based on what they think is best for the country or for their constituents. They do not pay attention to what their constituents want. It is sort of like how a parent is supposed to do what is best for a child, regardless of what the child wants. By contrast, the delegate style of representation, the legislator does what their constituents want. They vote based on those wants, regardless of what they think is best for the country.
It is not likely that there will be pressure on members of Congress to vote as trustees. The opposite is, in fact, true. If a member votes as a trustee, they may well be voted out of office by angry constituents. Therefore, there is pressure on them to vote as delegates. By contrast, if they vote as a delegate, they will likely be reelected. There is no pressure on them to vote as trustees so as to get reelected.