I went through this sutta today and made some notes quite rough but hope they are of use.
Twigs & leaves = gain, honour, & renown, this seams to indicate the positive worldly conditions so they are still moved by the world and firmly within it; unlikely to have any deep attainments but have either gained some form of understanding which they may or may not use to teach and gain support, but not necessarily essential as they may just be long in the homeless life and have things offered to them others may not receive due to age?
Outer bark = Virtue. Protected from the worldly conditions effecting them to an extent of breaking the precepts.
Inner Bark - concentration. not touched directly/effected all the time by the worldly conditions/hindrances.
Sapwood = Knowledge & vision. probably indicated someone who is a stream winner (maybe higher) as the Buddha specifically told the Stream winners to not be satisfied with the bliss of that attainment and to strive on with heedfulness in DN16. and they are protected from the extreme areas of the two extremes so do not go into the lower realms.
Negligence in the refrains does not seam to indicate they fall back from the level they are at, so they are knowledgeable or trained to a reasonable degree, virtuous, good meditators, have had insight... rather they are satisfied with this level of peace brought about by this knowledge... they have reached a plateau and don't look for more as they may believe they don't need to or that they are not capable of doing it?? or they may be fooled into thinking they have attained Enlightenment??
This sutta did remind me of another sutta where the Buddha uses the phrase "they are one in training still with more to do" but I can not remember the sutta, I did think it was MN53 the Sekha Sutta but I cheched and it isn't. although MN53 did also spring to mind in this reading.
He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them.
But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion …
He must be able to hear them from persons who actually believe them … he must know them in their most plausible and persuasive form.