Foster's Reactance Theorem - Reactance Increasing with Frequency?
Foster's Theorem uses reactance in the strict mathematical sense of the imaginary part of the impedance. This is of course signed. By convention, a capacitor's reactance is negative. At low frequency, it's large negative. At higher frequencies, it increases towards zero. This is opposite to the magnitude of the reactance, which of course decreases with increasing frequency. Read the whole of that wikipedia article carefully. What's wrong with the wiki article is it says 'It is easily seen ...', it can be seen, but not easily.



Image from the wikipedia article

These both show reactance increasing (becoming more positive) with frequency.