> Я - один из тех, кто всегда считал, что ub - это
> именно про поведение в рантайме. Можете кинуть ссылку на официальную информацию

Смотрите замечания, п.2 -- в том числе и в период исполнения.

ISO/IEC 9899:2017

3.4.3

1 undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which
this International Standard imposes no requirements

2 Note 1 to entry: Possible undefined behavior ranges from ignoring the situation completely with unpredictable results,
to behaving during translation or program execution in a documented manner characteristic of the environment (with or
without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic
message).

3 EXAMPLE An example of undefined behavior is the behavior on integer overflow.