Why command `vim -T dumb -c 'echo "Hello World!"' -c q` behaves differently when /dev/stdin is redirected from when /dev/stdin is not redirected?

Hi,

The following code runs well.

vim -T dumb -c 'echo "Hello World!"' -c q

~$ cat main.sh
#!/usr/bin/env bash

cat < /dev/stdin > /dev/null
vim -T dumb -c 'echo "Hello World!"' -c q

But running the following command will show "Vim: Warning: Input is
not from a terminal". The problem is related with /dev/stdin. Does any
know how to understand this behavior of vim? Thanks.

~$ ./main.sh <<< xxxx

--
Regards,
Peng

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