Here is a definition of open source software found on Wikipedia: "Open-source software (OSS) is computer software with its source code made available with a license in which the copyright holder provides the rights to study, change, and distribute the software to anyone and for any purpose".
I am assuming this is what you mean by "fully open-source". If you take a look at the Nordic license below, you can see that you can't just do whatever you want to with the Nordic mesh code. You have to abide by certain rules, and therefore I would argue that the Nordic mesh SDK is not fully open-source. However, if you follow the rules stated in the license, you can still do a lot of cool things that you could also do with fully open-source software.
Here is the Nordic license shown in the nrf_mesh_sdk.c file:
Copyright (c) 2010 - 2017, Nordic Semiconductor ASA
All rights reserved.
Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:
Redistributions of source code must retain the above copyright notice, this
list of conditions and the following disclaimer.
Redistributions in binary form, except as embedded into a Nordic
Semiconductor ASA integrated circuit in a product or a software update for
such product, must reproduce the above copyright notice, this list of
conditions and the following disclaimer in the documentation and/or other
materials provided with the distribution.
Neither the name of Nordic Semiconductor ASA nor the names of its
contributors may be used to endorse or promote products derived from this
software without specific prior written permission.
This software, with or without modification, must only be used with a
Nordic Semiconductor ASA integrated circuit.
Any software provided in binary form under this license must not be reverse
engineered, decompiled, modified and/or disassembled.
THIS SOFTWARE IS PROVIDED BY NORDIC SEMICONDUCTOR ASA "AS IS" AND ANY EXPRESS
OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY, NONINFRINGEMENT, AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL NORDIC SEMICONDUCTOR ASA OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
