I believe they refer to investors who have their portfolios managed by investment managers, such that the investors have an input into what assets they would like their funds to be allocated to, except in cases where the investment manager has full autonomy over the asset allocation, hence the feeling of regret will be on the part of the investment manager, if he/she failed to invest in bitcoin and it happened to perform well.

In either case, the author advises, that regret is to be factored in as a risk while allocating assets to mitigate its effect on the fund owner.