I want to know the meaning of “today's morphology is yesterday's syntax

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I guess yesterday’s syntactical of “all be it” and “on to” changed “albeit” and “onto” of today’s morpheme .

That's right. The insight behind the quote is that, over time, what used to be a normal construction created through productive syntactic rules eventually can become 'crystallized' into a single word or morpheme.

A good example is gonna. I quote the following from its Wikipedia page:

The going-to future originated by the extension of the spatial sense of the verb go to a temporal sense (a common change – the same phenomenon can be seen in the preposition before). The original construction involved physical movement with an intention, such as "I am going [outside] to harvest the crop." The location later became unnecessary, and the expression was reinterpreted to represent a near future.

This shows how gonna was originally a normal syntactic construction (be + VERB-ing + to). In contrast, in modern colloquial English, gonna patterns (in certain ways) as if it were a single word.