Basic doubt regarding "training" of a YOLO model
About Pre-trained model : This is a very common practise (especially in image recognition) and here is how we use it.
Let's imagine you want to recognize different types of food (beef, pork, vegetables, ...). You know some networks already exist that recognize all types of objects (boats, cars, food, sofas, ...). This objective of transfer learning is to use these models and adapt it to your task, so you pick an already trained model, and train it again, but only with your data (in our example types of food). Being able to recognize and distingish different objects is not useless for your task, so even if your network doesn't do the the same thing as the initial model, what the initial model has learned is used as foundations to learn your task.
We call this fine-tuning the model, it is basically using a complicated network that would be way too heavy to train because the dataset is not big enough, and just adjusting it so it performs our specialized task.
In your tutorial, the dataset Coco128 is made of 128 images, which is not enough for the training of a model as big as YOLOv5. So you use the pretrained weights and fine-tune it with your dataset. As far I as understand, the objective of the tutorial is to see how the model overfits these 128 images.

About validation, we usually splits datasets into 3 parts :

Training set : used to train the models. (~80% dataset)
Testing set : used to test the trained models, this part is the one used to estimate the accuracy of the model. (~15% dataset)
Validation set : used at the very end, to validate the fact that our model works correctly. (~5% dataset)

In many cases, Testing and validation datasets are the same (because validation dataset often is not necessary).
In your tutorial, what they call validation set is what I usually call the testing dataset, and it is the same as the training set, so bascially all your sets are the same. (it is very very very uncommon to do so, but since the goal of the tutorial is to see how we overfit this set, it makes sense here).