I've posted a detailed review
of the original article from the journal Antiquity
on my blog.
As at least a few others have noted the discovery is significant and fascinating but there is nothing to link the pre-Asokan layers with Buddhism. Indeed when you look closely there are other reasons to question the interpretation of the evidence. Tree shrines are ubiquitous in India from neolithic times to the present. They are far from limited to Buddhists.
The main evidence comes from a 50x50cm hole which is really rather small. The so-called tree shrine is off centre under the Asokan structure - its well defined edge (both at the paving level and the post hole level) goes right through the middle
of the Mayadevi Temple. A lot of the interpretation seems to depend on the idea that the centre of the tree shrine coincides with the Asokan structure. For example lack of roof tile shards in the dig are said to reflect an open centre. But the centres of the two structures do not coincide and this interpretation is thus wrong. Asokan monuments often expanded concentrically outward from existing monuments, but not here.
The question of why Asoka's architects would obliterate an existing tree shrine is not addressed. Compare the situation at Bodhgaya where the temple was build bedside the tree, not partially on top of it.
The post holes reflect unevenly spaced, oddly shaped, and irregular diametered posts. The idea that they represent a "fence" or "railing" is based on the presence of stone railings at Sanchi and Bodhgaya. But such stone railings as survive are evenly spaced, symmetrical, and regular. Thus from what we can see the post holes represent something almost completely unlike later railings. It looks more like a rough stockade than a carefully constructed shrine boundary.
The lack of alignment and the clearly rough and wonky nature of the fence that fits the post holes point to a discontinuity between that layer and the Asokan structure. Clearly humans had lived on the alluvial flood plain at Lumbini for some centuries before our best guesses for the lifetime of the Buddha. According to Buddhist historical narratives these were the Śākyas. Possibly originally from Iran
, the Śākyas were by definition not Buddhist.
These things seem pretty obvious and it is surprising that the authors of the article have ignored them. They have opted for a sensationalist approach - carefully hedged in the article, but uncritically in the media reports. No doubt many Buddhists will uncritically adopt the media reports as "proof" of their beliefs, but sadly they are being mislead.
We still do not know for sure when the Buddha lived and, despite the headlines, we have no reason whatever to review our current best guesses about the dates - different dates are accepted by different communities, but scholars now largely accept that the Buddha, if he lived at all, died ca. 400 BCE. This consensus has not changed the acceptance of other dates in various Buddhist traditions.