In both RPC commands `send()` and `walletcreatefundedpsbt` the RPC help was saying
that `add_inputs` default value was false when it's actually dynamically set
by the following statement:
`coin_control.m_allow_other_inputs = rawTx.vin.size() == 0;`
Which means that, by default, `add_inputs` is true unless there
was any pre-set input, in which case, the default is false.