Alright so we both agree it's not the literal interpretation. So in this case we have to seek God, context, and prayerful study to understand these harder passages. So I'll share my thoughts with you.
Okay notice what the author is focusing on in this passage...all these good life producing things... you've experienced the heavenly gift, you heard the word, you participated in all of these things... but you still don't have fruit? This type of person can not be lead to repentance, basically the idea is, they can't repent because they have experienced everything and it had no effect on them.
The overarching theme of Hebrews is not going back under the old sacrificial system and relying on Christ alone (even in the midst of persecutions).
Let's look at the next few verses to see this.
7For
ground that drinks the rain which often falls on it and brings forth vegetation
useful to those for whose sake it is also tilled, receives a blessing from God;
8but
if it yields thorns and thistles, it is
worthless and close to being cursed, and it ends up being burned.
Here's the author is contrasting different types of ground, one is useful and one is worthless.
Please notice that it didn't yield useful vegetation at one point, but that the focus is on it's worthless ground. Nothing can come out of it. So I don't believe this is a believer who falls away. We see this even more clearly in the next verse.
9But, beloved, we are convinced of better things concerning you, and
things that accompany salvation, though we are speaking in this way.
The author says, look I know I'm sounding harsh here, but I'm not saying this about YOU. I believe you are saved. And because you are saved the things that accompany salvation will be seen from you. And in fact have been seen from you in the past. You'll see he goes on to point those out in the next few verses.
His letter is to remind them they are saved. They are not worthless ground. And to continue to hold fast in what they believed. In my opinion, if they don't hold fast, well they never truly believed. His focus is to stay under the redemptive work of Christ and not fall under any other - especially after everything they've experienced and done so far.