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It’s a property of some operations and not others. exec just picks the column out of the table without walking through it. The equivalent operation is tab[;`col1]. However tab[`col1] is an odd operation because normally the first index to a table is the row and not the column, which implies that q must do something extra under the hood to come up with the same result as tab[;`col1], which includes rebuilding the column.
Similarly if you include a trivial condition in the exec such as exec col1 from tab where 1b then it acts the same as just exec col1 from tab . However if you add a nontrivial condition, even if it’s always true, such as exec col1 from tab where 0<=i then suddenly q has to walk through the column to build the answer which means it will be collapsed. Try different operations with t to see the difference in the time they take (I tried with 10000 rows and t:1000 to get a measurable result).