That picture shows "straight" legs, but with soft knees, not locked knees.
IMO, those things are not necessarily desirable, and are actually undesirable.
IMO, the big value of dead bugs is as a reflexive strength drill. Original Strength uses the definition of reflexive strength as "the body’s ability to anticipate, prepare, and respond to movement before and as it happens." In other words, it's patterns of muscle firing and relaxation that happen reflexively and subconsciously to facilitate movement and maintain stability during movement.
It's the opposite of (and complementary to) high tension conventional strength training where you are training the ability to consciously generate maximum tension. Doing a lot of aggressive bracing and generating high tension against high resistance is great for developing strength and building muscle. But it can lead to disrupting reflexive subconscious muscle firing patterns and creating spurious and excessive (and potentially painful) chronic tension when you don't need it.
You want reflexive strength drills, and most other mobility work for that matter (with exceptions for some specific techniques), to be relatively low load, low tension and low threat. The idea isn't to make the drill as hard as possible, but to keep it within a comfortable range. It's training the nervous system and neuromuscular connections and patterns more than pure muscular training.
