Computer Science is part of the Computer Science and Engineering Department, which is part of the Engineering Department. So, in order to be accepted into CS, you need to be accepted into Engineering.
Engineering has its own set of criteria that it applies to the admission process that goes beyond that of regular admissions to UB. Generally speaking, they are looking for stronger accomplishment in math and science.
What your decision tells you is that you have been accepted to UB, but not into engineering. I can only assume that means they felt your math/science background was not up to the requirements for engineering. However, as you appear to have found on the web site (https://www.eng.buffalo.edu/undergrad/admissions/current), it is possible to transfer into the department after meeting certain requirements as a general student. Read this carefully - there is more than the one way to transfer.
The “intending to major in engineering” phrase in your acceptance is an important one. This means they will (and you need to ensure they do) register you into engineering sections of early track math and science (Calculus, Chemistry, Physics). You must take the engineering versions of those classes - you won’t be accepted into engineering without them.
The classes you take will certainly count toward your CS degree. Is transfer automatic? If you meet the criteria as documented, you will be admitted. If you are truly concerned, I would email the engineering department to confirm this, and hold on to that confirmation, but I’m sure they stick to their policies. I imagine you will need to administratively apply in some manner, but if you qualify, you qualify.
Is taking the transfer path risky? It can be, yes. However, if you fail to meet the qualifications for transferring, I would suggest you were likely at risk for not succeeding in CS to begin with. As you note, for one of the paths into the department, you will need to maintain a 2.5 average in your major in at least three courses in CS. Since these are likely to be entry level courses, if you are struggling with those, perhaps the major wasn’t meant for you.
Should you pursue this option? There is no easy or obvious answer. That is completely dependent on you, your confidence in your abilities, and your other options. Do you feel strongly that UB is the right place for you? Do you feel your math and science are stronger than your grades and test scores indicate, or how UB Engineering has interpreted them, and that you can do well? Do you have viable offers from other schools that have taken you directly into CS? If you go to UB, but cannot transfer into CS, are there other paths/majors that interest you?
These are all hard questions, but ones you have to answer. Best of luck with your decision, and congratulations on your acceptance as a general student.