Lenovo released this source code on 4 June 2020.
According to Lenovo, the same kernel works for the TB-X605F (wifi) and TB-X605L (LTE) variants.
The code is msm-3.18.120, which is the same version as included in the offically-released stock ROM for Android Pie.
To compile the kernel:
export PATH=$PWD/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/bin/:$PATH
rm -rf out/
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-android- O=out m10_msmcortex_defconfig
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-android- O=out -j8
The kernel is built to: out/arch/arm64/boot/image.gz