I'd say it's due to REM rebound and many other factors.

The reason for the WBTB is to really help elevate awareness a bit, since you're disrupting (ideally and hopefully) the onset of a REM period or the end of a REM period rather than going through the whole sleeping periods while most likely being unconscious.

I would presume the people who did WILD without a WBTB is that somehow, they were able to be conscious of when they woke up during an interval at night, and just happened to close their eyes back and go into a dream, since they will up for a few seconds feeling how it's like to have their body relaxed, which is a huge help for falling asleep.

Another reason, though it's a controversial one, is how they conceptualize a WILD and Beyond Dreaming aspects like AP/OBE. It depends on what each person thinks, but those who claim to have done an AP/OBE would be viewed as a vivid WILD to others.

Other reasons could be their mindset, supplements that may have suppressed N-REM periods (by a little bit) to have sudden bursts of longer and stronger REM cycles as compensation.