This link https://www.sitecore.com/knowledge-center/getting-started/training is still pointing to old URLs and will redirect you to the login page of the new Sitecore learning portal. The Sitecore team needs to update the content here, hoping they will do it soon.
Now with regards to your other questions -
Do we need to log in to see the content of the course before purchasing?- Yes, Sitecore's new learning portal's default page is the login page, so you need to log in first before doing anything on this portal. you can click any of the login types which best fit you. Sitecore's login uses OKTA so once you will do successful login you will be able to see the learning home page.
If someone doesn't have login credentials then how they can see the course material? - If someone is not having logins he/she needs to create an account on https://www.sitecore.com/account/signup it asks for a business email but will work for your Gmail or other email ids as well. once signup will be complete you will receive an email to set up your login password
Alternative of https://learning.sitecore.com/pathway/sitecore-developer-bundle - Once you will log in to the learning portal, on the home page you can browse for the learning catalog, which will give you a complete list of available products. You will be able to see the Sitecore bundle there. once you will log in you can run this URL as well - https://shop.learning.sitecore.com/products/bb21d39f-8a39-d1 (but this will not work properly if you are not logged in, it will redirect you to a login page which will not work as shop.learning website is using token-based authorization.
But this is a new portal and things are still going on you will not get everything that was in the old learning portal.
