Semiotics claims to be older than memetics. Semiotics became popular in the 1970s and 1980s - but claims roots going beck centuries. However, until the 1970s there wasn't very much in the way of semiotics publications. The founders of the various schools of cultural evolution may have ignored semiotics, because it had yet to become popular at that time.
By the 1970s, semiotics had basically failed to produce a school of cultural evolution. There was no explanation of how signs evolved based on broadly Darwinian principles. As Terry Deacon put it:
Until now, classic semiotic theories have not had much to say about why certain signs persist and others do not, or why certain semiotic systems evolved the forms they now exhibit....and...
The meme concept has generated recent excitement precisely because it seems to offer hope of providing something that other theories of social and semiotic processes have not succeeded in providing. It addresses the process of semiosis, i.e., the dynamical logic of how the symbolic and concrete constituents of culture arise, assume the forms they assume, and evolve and change over time.
Retrospectively, we can see that application of evolutionary biology to human communication mostly arose outside of semiotics - mainly from those trained in evolutionary biology and population genetics.
Perhaps the bypassing of the term 'sign' by cultural evolutionists was inevitable. The term 'sign' - in common parlance - comes with an associated object that is signified by the sign. Culture contains many signs - for example, letters, words and ideograms. However there are also non-signs: for example, knots, cups and fire. These have no obvious referents - they just are. For the concept of 'sign' to be useful as a unit to cultural evolutionists all culture would need to be signs. However, that violates the common dictionary definition of 'sign'.
The tpyical semiotics solution to this problem is to expand the definition of 'sign' to include knots, cups and fire - and indeed, practically anything. This is sometimes called 'pansemiotics'. If you do this, then semiotics becomes very general. Of course the problem is then that the original meaning of the term 'sign' has got lost. It is sometimes permissible to give common language terms counter-intuitive technical meanings. However, here, I think it just leads to pointless confusion.
As for the claim that the concept of 'meme' misses out the concepts of semantics and observation: this is just sour grapes on the part of the semiotics folk. One might reply that meaning and observers aren't part of the meme because they are context-dependent.