It's likely that Yoda actually did keep in touch.
We can't say for sure that Yoda did or did not stay in touch, given that the new canon that's currently being written, but Yoda has been shown to be quite capable of contacting people via the Force.
Moreover, we don't know what exactly happened after the end of Revenge of the Sith; Yoda claims that he will teach Obi Wan how to commune with Qui-Gon:
The secret of the Ancient Order of the Whills, he studied. How to commune with him. I will teach you.
and, given Kenobi's success in becoming a Force Ghost, it can be assumed that this spirit speaking training was successful. We then have a 16 year gap between this point and A New Hope, wherein we learn (in the above video) that Yoda has the ability to keep in touch. Thus, it can be assumed that at some point Yoda either met with Obi-Wan or contacted him through other means.