I was trying to reproduce codes used in the above git repo, here is what I found.
python -c 'import tensorflow; print(tensorflow.__version__)'

> 2.4.1

>>> from keras.engine.keras_tensor import KerasTensor  # imported from keras-team/keras
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'keras'
>>> 
>>> from tensorflow.python.keras.engine.keras_tensor import KerasTensor as KerasTensorFromTF # This import should not exist anymore
2022-01-28 10:19:31.459850: I tensorflow/stream_executor/platform/default/dso_loader.cc:49] Successfully opened dynamic library libcudart.so.11.0
>>> 
>>> assert KerasTensorFromTF == KerasTensor
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'KerasTensor' is not defined

>>> import tensorflow as tf
>>> 
>>> from tensorflow.keras.backend import is_keras_tensor
>>> from tensorflow.python.keras.backend import is_keras_tensor as is_keras_tensor_tf  # this import should not exist anymore
>>> 
>>> assert is_keras_tensor(tf.keras.Input([10]))
>>> assert is_keras_tensor_tf(tf.keras.Input([10]))

TF 2.4.1 has no issue
While TF V 2.7.0
$ python -c 'import tensorflow; print(tensorflow.__version__)'
2.7.0

>>> from keras.engine.keras_tensor import KerasTensor  # imported from keras-team/keras
>>> 
>>> from tensorflow.python.keras.engine.keras_tensor import KerasTensor as KerasTensorFromTF # This import should not exist anymore
>>> 
>>> assert KerasTensorFromTF == KerasTensor
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

>>> import tensorflow as tf
>>> 
>>> from tensorflow.keras.backend import is_keras_tensor
>>> from tensorflow.python.keras.backend import is_keras_tensor as is_keras_tensor_tf  # this import should not exist anymore
>>> 
>>> assert is_keras_tensor(tf.keras.Input([10]))
>>> assert is_keras_tensor_tf(tf.keras.Input([10]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/ccs/home/asd/.conda/envs/gdy_v2/lib/python3.9/site-packages/tensorflow/python/keras/backend.py", line 1281, in is_keras_tensor
    raise ValueError('Unexpectedly found an instance of type `' + str(type(x)) +
ValueError: Unexpectedly found an instance of type `<class 'keras.engine.keras_tensor.KerasTensor'>`. Expected a symbolic tensor instance.