[intro.execution] p8 strictly states that

Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a single thread ([intro.multithread]), which induces a partial order among those evaluations.

In other words, the "sequenced before" is based on two "evaluations". We can note that we use "execute" in the whole of clause [stmt.stmt], such as:

Except as indicated, statements are executed in sequence.

If the condition ([stmt.select]) yields true the first substatement is executed.

and so on.

An unclear point by using "sequenced before" is in [basic.start.dynamic] p5

It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with static storage duration is sequenced before the first statement of main or is deferred.

statements are said to be executed while expressions are said to be evaluated. The wording in [intro.execution] p8 is more and less implies that the "sequenced before" is applied to two expressions, or at least to the language construct that is evaluated. So, the issue arises from the meaning between "execution" and "evaluation".