Not true at all. If you are generating quality post, you will get merit. No matter it’s a reply or a thread itself. Without quality, no matter what you create, you are unlikely to get merit.
Regarding creating thread, I see no problem with that if the topic is something to discuss about, if the topic isn’t discussed hundreds of times. Then there's nothing wrong to create a thread.
I think its somewhat true, maybe less these days, but when merit was initially introduced, and we had less merit sources it was definitely the case. Although, I do think it still applies to an extent. In terms of exposure, users will usually read the first post, and maybe a few posts there on after, but when there's multiple pages the odds of users browsing all the replies diminishes somewhat. Of course, merit sources probably do read a lot more replies than the average user, but I think there's quite clearly an issue sometimes with users not reading the entire thread, and thus the probability of a post getting merit also diminishes with that.
A thread will likely always get more exposure than the replies, and therefore likely from a statistic point of view, will generally receive more merits. Not always the case of course, but in general I think that would likely be the case. Obviously, without any good statistics available its hard to prove.