I’m more forgiving of advertisements. They’re carefully crafted to create an overall effect, and if the niceties of grammar interfere with that, the grammar quite rightly (in terms of the advertiser’s objective) goes out the window.
Here is the ad in question.
First, it must be stated there is no real ambiguity in the construction. No one seriously thinks they were talking about a heart that weighed a ton. The intended meaning is clear. So the question is merely that of form, not of substance.
There’s a rhythm to the phrase of an average horse. But inserting the extra syllables--and this is a spoken phrase, not a written one--required for the standard grammar (of an average horse’s or of that of an average horse) disrupts that cadence. It’s less mellifluous and sounds more stilted, less natural. (Which gets me wondering how common this construction is in ordinary speech--it may not even be incorrect at all.) Just about the worst thing you can do in an ad is have stilted language that calls attention to grammar rather than the message. Yes, us grammar nerds may notice, but we’re a small minority.
An alternative explanation is that hypercorrection is at play here. The advertisers might be trying to avoid a double genitive. Of course, heart of an average horse’s is not a double genitive, but it has the same form of one, as in father of Kim’s.
Nitpick: the ad says average, not normal, not that it makes a difference to the grammatical analysis.
And here’s an article on the truth of the claim that Secretariat’s heart was twice average size. In summary, it is likely that it was larger than normal (not unusual among champion thoroughbreds), but probably not twice the size. The vet who performed the necropsy and is the source of the claim did not actually weigh Secretariat’s heart. He just estimated its weight from visual inspection.