I came up with a few things that might explain it. I don't think the changes wrought will necessarily result in a "thin" beer, but it's hard to say exactly what thin means anyway, so here goes.
Low wort pH, though not the most important factor in final beer pH, does have an effect. So a more acidic wort should produce a slightly more acidic beer, all other things being equal. Does acidity = thin? Not really sure, but it will definitely affect the mouthfeel somewhat.
Lower wort pH leads to a lower utilization of hops in the kettle, especially with regards to bitterness. If you have a utilization number you're used to on your system and you throw in a wort at a lower pH, you may end up with a beer that has less bitterness than you'd normally expect. Would it taste thin? Again, not sure...
Low wort pH also reduces the ability of the beer to clarify properly in the kettle (to form hot break that will settle out effectively). This could lead to differences in fermentation triggered by presence of excess trub in the fermenter and also possibly to excess polyphenols (which taste astringent) that would otherwise have left the wort as hot-break. About kettle clarification, Charlie Bamforth says "[w]orts below pH 4.5 completely fail to fine, and a pH of 5.0 is needed for effecient settling".
Regarding the first point above, if you carry over a bunch of excess trub to the fermenter, the idea is that it provides nucleation sites for CO2 in the fermenting beer to come out of solution. CO2 has an inhibitory effect on yeast, so more trub (and therefore less CO2 in solution) should stimulate higher rates of yeast growth and metabolism. Factors which favor higher rates of yeast growth tend to lead to both lower finished beer pH and lower levels of esters, which could make for a less full-tasting beer. I feel like I've definitely heard people before suggest the link between rapid fermentation and thin tasting beer.
Lastly, lower wort pH at pitching can change the fermentation flavor profile the yeast will produce. It has been shown to lower the formation of DMS during fermentation, which in small quantities can add a distinctive body to some beers. The lower pH also aids in more rapid diacetyl reduction to flavor-neutral compounds. This could be important to the perceived body, particularly in British-style ales.
OK, a lot of this is a bit of a stretch, I know. But I have to say I had fun looking around trying to find effects of wort pH that didn't involve mashing/enzymes. Hope some of it tickles your fancy.