Surge Arrester "Low Current" In Pressure Relief.

In what context?

I think there's a language problem here. Pressure relief refers to fluids under pressure. Nothing to do with electrical engineering.

It has everything to do with electrical engineering of the application of a surge arrester. Gas is a fluid and in this context it is the internal pressure generated as the surge arrester heats up as it is conducting the surge to ground.

From a Selection Guide by GE:

"...In the unlikely case of complete failure of an arrester, a line-ground arc will develop and pressure will build up inside the housing. This pressure will be safely vented to the outside and an external arc will be established provided the fault current is within the pressure relief fault current capability of the arrester. This low-voltage arc maintains equipment protection. Once an arrester has safely vented, it no longer possesses its pressure relief/fault current capability and should be replaced immediately. For a given application, the arrester selected should have a pressure/fault current capability greater than maximum short-circuit current available at the intended arrester location..."

The OP's question is also answered there.

The second sentence is only partly true. There is very little heat or internal pressure generated during normal arrester operation in conducting overvoltage surge current to ground, followed by normal reseal. The buildup of internal pressure and pressure relief is associated with fault current in the event of arrester failure. Arrester failures are typically short circuit so one example in which pressure relief would be critical is circuit breaker or switch close in on a failed arrester.

OK, I thought he was confusing 2 different pieces of kit

It depends on the class and construction of the surge arrester but surge arrester standards generally require both high and low current pressure relief tests and ratings for station and intermediate class arresters. Symmetrical rms pressure relief test currents and ratings for station class are typically 40kA - 65kA (high) and 600A (low) and 16.1kA (high) and 600A (low) for intermediate class.

Typical arrester designs will have a two stage pressure relief system to control and prevent the build up of internal pressure. A low current fault may only cause a slight increase in internal pressure, which will initiate a low pressure relief operation, allowing the venting of the excess gas pressure in a safe manner while a high current fault results in a rapid increase in internal pressure which must be rapidly relieved to prevent catastrophic failure of porcelain housings, for example.