A text published on LinkedIn by Katerina Xafis:
DA and TA were different because of the definition of 'discourse' and 'text'. In the early 70s, 'discourse' (ie spoken discourse)was defined as a process involving coherence, while Text (ie written discourse) was viewed as a product involving cohesion. Towards the end of the 70s Widdowson withdrew his support of this view by stating that discourse consists of sentences (SFG's clause complex) that entail both cohesion and coherence.
Another distinction between 'text' and 'discourse' was the view that meaning was found in discourse but not in text. (Perhaps Matt the prof was referring to this meaning of 'text' when he said DA was not directly involved in meaning?) Again this distinction was found not to be watertight given reader interaction with written texts to extract meaning, so eventually the term 'text' and 'text analysis' were replaced with 'discourse' and 'discourse analysis', removing a lot of unnecessary confusion in the process.