no it wasn't a myth
rather than pay players they were to be compensated for time lost at work on game day-people in industry worked on Saturday. It was a fixed ammount.
By this the northern clubs thought they would stave off vprofessionalism and stay onside with the RFU. The people running the clubs weren't socialist firebrands, they were figures of the establishment
You don't have to be a socialist firebrand to start a rebellion. The rich men running the game in the north were mostly nouveau riche with their money coming from the industrial revolution. The people running the RFU were the ancien regime and felt threatened by working class players busting their patrician a**s and petit bourgoisie funding these people making the patrician rulers of the game and it's founders very defintely at the bottom of the pecking order.
They seized upon the broken time issue as a club with which to repel this invasion of working class players and get them out of the game. They were quite happy with gentlemen players claiming extravagant expenses for playing but broken time was to be verboten.
The Northern clubs did not so much paint themselves into a corner as they were forced into it by the RFU who were hell bent on getting the balance of rugby power back from Wigan and Leeds to Harlequins and Blackheath and were prepared to commit rugby suicide to cut this cancer of Northern dominance based on working calss players out of their game.
Faced with that the good burgers who ran the Northern Union clubs did indeed rebel against the oppression that was being foistered on them by the RFU.
The whole situation has a similarity with the American revolution. They were being dictated to and discriminated against by the British and broke away and broke away/rebelled to safeguard their way of life but they weren't wild revolutioanries. The state they founded had a property vote only, so half the population could not vote and they retained slavery. They were men of property and business but they rebelled nevertheless.