Is ACS712 isolation sufficient for SELV?
Welcome to the ambit claims &amp; half-truths of electronic component datasheets :-(

I belive you're correct that, depending on the contamination grade you're aiming for, this device on a solid PCB won't meet your requirements.

Adding a slot in the PCB, along with appropriate PCB routing, will help in general (without reference to any specific standard you're trying to meet) by increasing the creepage distance, but I don't think you want to be filling the slot with glue, unless you're meaning to add a 'wall' of glue in the Z plane all around the top-side of the device to increase both clearance and creepage, which is (a) ugly, (b) very difficult to service, &amp; c) probably more expensive to manufacturer in a repeatable manner than a more appropriate selection of device.