The confusion is because the meaning is lost in translation. Let me rephrase it:
Swans extract sap “milk”from the stems of water plants and have filters (lamellae) in their mouth to keep out dirt and mud while doing so.
In conclusion, if it is described in the Vedas, it must be true.
Regarding crows they might be materially successful by consuming all kind of nasty things. But scriptures advise to not consume whatever filth one comes across. Instead have the ability to filter out the impurities and only take the essence like a swan who takes pure sap while leaving behind all the mud.
