Yes, absolutely. These are sometimes used for high power loudspeakers.
I think for the dimensions of the connector itself, 15A is pretty reasonable, and for the application, sometimes decently powered audio connections. 14 guage wire is only 2mm\$^2\$ and good for 15A AC in free air. A quarter inch connector has 31.668 mm\$^2\$ to work with. With modern insulation you could probably design one to pipe a lot more than 15A or a lot more than 240V depending on whether you wanted to use the extra space on conductor or insulation. Even a 3.5mm phone jack has 9.62mm^2 to work with and could probably make 15A @240V with the right design.
Often specific purpose connectors seem to be rated for the purpose rather than the actual theoretical maximum. In order for the pin to be adequately mechanically robust, a certain amount of metal ends up getting used, and simply balancing out the cross sectional areas of the tube and inner pin would yield a pretty decent current rating, and even more so for the female connector. The original use was for a telephone switchboard so they had to be able to take a beating. I would imagine the choke point becomes the spring loaded contacts.
Phone connectors have been around for so long (1877) and are both relatively versatile and safe for lab and hobby connections. Because they were ubiquitous and cheap when many other connectors weren't, it probably didn't take too long for them to be used in non-audio applications enough for maximum current ratings to be established.