I set up a new ubuntu 20.04
and did the following steps:

sudo apt-get install git
sudo apt-get install python-is-python3
sudo apt-get install python3-pip
sudo apt install curl
curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -
echo "deb https://packages.cloud.google.com/apt coral-edgetpu-stable main" | sudo tee /etc/apt/sources.list.d/coral-edgetpu.list
sudo apt-get update
sudo apt-get install edgetpu-compiler

pip3 install numpy
pip3 install keras
pip3 install tensorflow

git clone https://github.com/guichristmann/edge-tpu-tiny-yolo.git
git clone https://github.com/guichristmann/keras-yolo3.git

after that I copied the coco-tiny-v3-relu.cfg and the coco-tiny-v3-relu.weights from the edge-tpu-tiny-yolo dir to the keras-yolo3 dir
and did a python convert.py coco-tiny-v3-relu.cfg coco-tiny-v3-relu.weights mytest1_keras.h5
I copied this new h5 file back to edge-tpu-tiny-yolo and did a
python keras_to_tflite_quant.py mytest1_keras.h5 mytest1.tflite
after that I did an edgetpu_compiler mytest1.tflite
but I got the well hatet "Model not quantized" message I got with my own weights, too

So something changed since you set up your scripts.
The also provided original quant_coco-tiny-v3-relu.tflite is compiled by edgetpu, so the problem must be
earlier.
Please check your own files and tell me, what to do.

Best regards
Roland