The basic problem with these statistics are that they don't tell anything about who is being polled. While the American Baptists are a specific denomination (separate from the larger Southern Baptists), and the Episcopalians might be identified as a denomination, there is not a single "Presbyterian" denomination. There are at least a dozen, ranging from flaming liberals to ultra conservatives. Likewise, there are at least 3 major Lutheran bodies (none of which is "American Lutheran"), and even several Methodists. From the description, it is hard to tell if the survey actually supports what the numbers purport to say.
True, it'd be nice if we knew where he sent the 10k surveys, but it was done in 1987.
Regardless of the details, it's still alarming, imo.
More recent surveys of average church goers and members tell a similar story. If you google similar keywords, you'll find that the majority of those who call themselves Christians don't believe in the divine inspiration of scripture, among other things.
And if that's what they believe, you have to wonder what they're being taught. I'd love to see a more recent survey done of clergy, but I'd probably be appalled at the results.