I consider that this somehow is quite an interesting feature.
I find that the “heart” icon is much better suited for marking really important posts and retaining some sort of bookmark.
However creating a new additional mode “thumbs up” mode is more of an approval or notification or just inform about your status (similar to the bump feature messengers have). When you say for example “OK I read your message”.
So for example if you mark posts as favorite you can bloat your real favorite posts, if you just respond in a new post you can just bloat the thread with no meaningful information. So the final point of this new mode is the solution to these two separate things.
I don’t know if you have though any of this or perhaps am I just “knit picking”, just an idea is what I am mentioning.
P.S. However this suggests that many users find this idea good and the message is passed down to the forum developers and then see how it goes.