Was the influence of Taoism on Buddhism responsible for the development of Mahayana?
It is hard to assess half a sentence without a wider context, but I would interpret this as "it was the influence of Taoism that was responsible for the development [but not creation] of [some branches of] Mahayana[, more specifically, some of the Chinese ones]". And in this form the statement is true.

You seem to interpret it as "it was the influence of Taoism that was responsible for the development of Mahayana [in India, even before it reached the Chinese soil]". Which is a false statement.