None of the references actually explain this properly. To understand this usage, which is literary, some background is necessary. Classical Japanese has two copulas, にある and とある, which were redued to なる and たる, and these two copulas were used to form two types of 形容動詞, なり活用 and たり活用.
The modern な adjectives derive from the なり活用の形容動詞 (形動なり), for example,
静かなる ➞ 静かな
遥かなる ➞ 遥かな
They are formed largely (but not exclusively) from native Japanese words, as above, and you will still encounter the older forms in written Japanese, and songs, where some kind of elegant, literary flavour is sought.
The たり活用の形容動詞 (形動たり) have only survived in literary Japanese, but occasionally may be heard:
興味深々たるものがある 'There are things full of interest.'
純然たる片田舎で 'in a really (purely/unadulterated) out-of-the-way place'
and they are largely formed from two-character compounds.
Now, 形動なり, and thus な-adjectives, are adverbialized by changing な to に:
while 形動たり are adverbialized with と:
二人は身をかくそうともしないで、通りの真中を堂々と歩いていった 'The pair of them, with no attempt at concealment, walked boldy along the middle of the road.'
It is also possible to make these 形動たり sound less formal by substituting とする for とある:
堂々とした風采の人 'An imposing person.'
これだけの死者を行政はどう思っているのかと暗澹たる気分になる One gets gloomy, wondering what the government thinks of this many deaths.
この暢気な連中をどうやって守るかを考えると暗澹とした気分になった When I considered how I was to protect a carefree crowd like this, I felt gloomy.
To finally answer your question, the writer has adverbialized 徒花たる, yielding 徒花と.
'As' is an accurate translation of と in these instances.