Why is OPT at least the most valuable item for FPTAS Knapsack?
It is common to assume without loss of generality that the maximum item weight is at most as large as the knapsack capacity. That is okay (in the context of complexity theory) because filtering out items that are too large to fit even alone is easily possible in a polynomial-time preprocessing step.

If the sources do not mention that they are sloppy.

And in that case, every item fits into the knapsack alone, hence you can obtain the maximum item value with a trivial singleton solution.

Nota bene: As soon as we are talking algorithms and want to derive running times, such assumptions are not necessarily w.l.o.g. In case the running time cost of the preprocessing is dominant we should not lie it away.