Al the otto plecos are algae specialists. Most omnivore plecs will nibble at the glass when bored. Chaeostoma are great. My L200 is mature and still loves cleaning the glass for me. So I'd agree that you received false info.
<schpiel> Of course, you ahould never expect any pleco to just live on algae and fallen food. They are fish in their own right and need a balanced diet like any fish.</schpiel>
if you have an excess of algae on the walls, and no fish that might scare a plec, than they might hang out on the glass a lot. but mine seems to enjoy the driftwood, its all about the individual.
I have never seen green on the glass of any tanks that had plecos in them fyi.they will clean for you. seem to slow down as they get older around a foot long now the common and high fin /sail fin getting lazy. lol
it was bad intel . . . I second an earlier comment, otocinclus cats and chaetostoma - - bulldog/rubber plecos - - will spend much (most) of their time on the glass . . . I keep otos in my small tanks, bulldogs in the larger ones (and sometimes both)
but for sure, you need to supplement their diets, they could starve otherwise . . .