I'd probably make Edberg the 60-40 favourite to win a hypothetical Wimbledon final against Lendl in 1989. Lendl had won their biggest grass court match (clearly their 1987 Wimbledon SF was a bigger deal than their 1985 Australian Open SF), but Edberg had won 3 majors on grass by that stage most notably of course Wimbledon the previous year, while Lendl hadn't won a set in a major final on the surface.
Plus clearly both players would serve volley behind 100% of 1st and 2nd serves, so I'd have to favour the player far more naturally suited to that style under that scenario, with the noticeably better movement and footing on the surface to boot, and also clearly the better return of serve on grass. And as has been previously said, there's no guarantee that Edberg would get off to same slow start vs. Lendl as he did vs. Becker, and he did play very well during his SF win against McEnroe in what was a serve volleying masterclass by both players. And Lendl's nerves and desperation to win that elusive prize that he so desperately wanted, would surely be a factor.
Their match-up was pretty even on carpet and hard courts at that stage (with them splitting wins against each other on both of those surfaces that year), but on grass I'd have to give Edberg the advantage. Lendl beating Becker and Edberg back to back to win a major on hard courts would be tough enough, let alone at Wimbledon on grass. When past his prime he nearly beat them back to back merely to reach the US Open semis in 1992 after coming from 2 sets to 1 down to beat Becker in the 4th round, and then coming from 2 sets to love down before leading Edberg by a break in the 5th set of their QF, but he couldn't quite pull it off.
Now at the 1990 Australian Open, if Edberg hadn't suffered that abdominal injury during his final service game of his flawless SF demolition of Wilander, I'd say that going into it his final against Lendl would have been pretty much 50-50.