I have the Japanese version too. It's not on the postscript itself but at the bottom of the same page where there is a small drawing of Yoh. The text is very small and they are both side of the Yoh's drawing.
I took some pictures of that section. I don't know if this works.
http://www.mangago.me/home/photo/10938145/
http://www.mangago.me/home/photo/10938147/
http://www.mangago.me/home/photo/10938148/
Sorry I ignored that information. I’ve double checked and find out the “付き合い” means “privately maintain contacts”, not “privately lovers”. So I think the postscript should be translate as “the two of them privately maintain contacts”. Sensei also declared that due to the unequally postion, yoh and Fei are difficult to develop a romantic relationship. I feel pity about that too, Yoh is a nice guy ^ ^
This information is correct. The English definition of lover is: "a partner in a sexual or romantic relationship outside marriage." Fei Long and Yoh absolutely were lovers in FnR, and it appears they've maintained some kind of relationship since then. However, it's purely sexual on Fei's part, as he does not have romantic feelings toward Yoh. Mostly it's because he still has strong feelings for Asami. Yes, Fei uses sex as a weapon when it suits him. He was trained to do that back when Yan had a hold over him.
In my version (french) it is said that after Yoh confessed, they start being very close but it is not clearly said they are dating or are in love with each other. Maybe I'm wrong but I think Yoh accepts the fact that Fei Long's heart is out of reach so being able to be with him and maybe having sex with him sometime is sufficient. This is why he also accepts the fact Fei Long can have sex with someone else.

I just received my copy of volume 10 and noticed at the bottom corner after the author's message, there is a drawing of Yoh (Ye) with the description "Ye, Fei Long's former subordinate. Originally Asami's spy but fell for Fei long, with his cover bown, he couldn't stay with Fei Long or go back to Asami. So he want independent. From Hong Kong, but reside in Taiwan, will wait for day for Fei Long. Recently got a haircut. His confession to Fei Long didn't go unrewarded. The two are now privately lovers."
What?? Aside a side note, in volume 9, Yamane-sensei said she likes their ambiguous relationship. What happened??
I like to see Yoh and Fei to be together but reading this, I feel bad for Yoh now. Yoh knew Fei slept with Mikhail the first time which I can justified Fei Long views sex as a weapon which he used. But the second time when they both have trouble sleeping, that seems like an emotional request which you think you should rely on your lover. I don't know, maybe Yoh is that giving when it comes to Fei Long.