DL's "correct" answer sounds a little off, but I can't explain why. If you want to use "have" then it would sound better as "they have understood it all." But, you don't need the "have." It sounds best as "they understood everything." Is there any reason why DL is inserting "have" into this sentence?
In colloquial American English, whether you use "have understood" or "understood", you will more likely encounter "everything" as the direct object. If using "all", then you'd probably encounter "understood all of it" rather than "understood it all". But all versions are correct and not unusual.
What I don't understand is why "have understood" is used to translate Perfective aspect. "Understood" alone indicates a definite moment in the past when they understood everything - and probably still do understand, once having gained that understanding. "Have understood" has a connotation that they might not understand now, even though they understood in the past - but they might also still understand, which seems to lend itself more to Imperfective, as least so far as I've been able to grasp the differences between the two aspects. That part of Russian grammar is still very much a work in progress for me.
"According to me" sounds as though you are referencing yourself as an expert on the subject, e.g. "Oh yeah? According to who?" "According to ME." While the same connotation could be applied to "in my opinion," the latter reads more as an assessment, uninformed or informed, and less a declaration of authority.