First, kudos to you for reading the instructions!!! :dance:
Second, there are usually weight limits on the top tether anchor provided by the car manufacturer. Many of us feel confident in ignoring that weight limit, as it's assumed the tether anchor is very unlikely to break and/or the tether itself is unlikely to break and by the time it does, it will have already served it's purpose (limiting the forward motion of the child's head).
Another consideration is the younger the child, the more at risk they are. Often by the time a child is old enough to weigh 65 lbs, they are a bit sturdier. My 10 year old is a little below average in size and he's 65 lbs. So, one could argue that it is less necessary in many larger kids b/c they are likely older. As long as my kids were harnessed, I always used the top tether where one was available. Granted, my kids outgrew their harnessed seats by height long before they reached the weight limits, but most kids do.
Many vehicles can have top tether anchors retrofitted. If you want to share your vehicle's make, model, and year, I'm sure someone here can help you.