Interesting article, @John_Dalton. I bet @Jimpithecus can enlighten us a bit more on the subject, but here's my take. The reason this is of interest is because of the light it sheds on the evolution of the brain and "modern" human cognition. For instance, H. heidelbergensis was using spears 300,000 years ago to hunt large game, particularly horses, but their spears were essentially sharpened sticks. "Hafted" tools (a stone head attached to a wood handle) are much more complex to make, and if an adhesive is used, that adds yet another layer of complexity, since the making of the adhesive is yet another multi-step process.
Your guess is essentially correct, @GJDS. Tar wasn't found associated with H. sapiens tools because, well, there are no birch trees in Africa. Simple adhesives, such as tar, are ancient, but compound adhesives mixing multiple ingredients do not appear until later, perhaps 70,000 years ago. Here's how a paper on Implications for complex cognition from the hafting of tools with compound adhesives in the Middle Stone Age, South Africa describes it:
"The use of simple (1-component) adhesives is ancient; for example, birch-bark tar was found on 2 flakes from ≈200,000 years (200 ka) ago at a site in Italy (3). At ≈40 ka, bitumen was found on stone tools in Syria (4), and a similarly aged site in Kenya yielded tools with red ochre stains that imply the use of multicomponent glue (5). Traces of even earlier (≈70 ka) compound adhesives occur, together with microfractures consistent with hafting, on Middle Stone Age (MSA) stone tools from Sibudu Cave, South Africa (see SI Text and Table S1). Several recipes are evident: sometimes plant gum and red ochre (natural iron oxide–hematite–Fe2O3) traces (Fig. 1) occur on tool portions that were once inserted in hafts (6⇓⇓⇓–10). Other tools have brown plant gums and black or white fat, but no ochre (Fig. 1 and SI Text)."
The ochre made the plant gums less brittle, apparently. Hafting of stone tools by both humans and Neanderthals was previously thought to have begun between 200-300,000 years ago, but one recent paper on Early Hafted Hunting Technology pushes that date back to 500,000 years ago.