Seymour - a fanatic for flat tax - saw two words. They were dirty words and they smelled like communism: "income redistribution."
Seymour was quite justified in flagging this. After all, what Joyce described as "income redistribution" was what National would normally call "bracket creep".
National would normally see that as a problem, not something to gloat about.
Come election time it will undoubtedly again see it as a problem when it sets about persuading those taxpayers that they deserve the tax cuts National has on offer.
It was not long before Joyce committed another sin in the book of Act: he gave out some grants to small businesses. This, Seymour said, was "corporate welfare".
Then last week, Joyce further compounded his crimes by dismissing a Productivity Commission recommendation to reintroduce interest to student loans. Seymour accused the Government of being too chicken to even debate it: "Muldoon lives!"
By now Joyce had quite an impressive tally of alternative titles to his name. He was Minister of Corporate Welfare, Minister of Communism and Muldoon.
The reason Key gave for ruling out interest on loans was the same he gave back in 2008, when he adopted Labour's interest-free policy in the first place: if he put interest back on to student loans he would lose the next election.
According to Seymour, this might not be a bad thing given the Government was veering wildly left. "What is even the point of electing a National government?" he asked.
Then along came a reminder for Seymour of what else was in the balance should National get voted out. Himself. The reminder was delivered in the form of the Green Party decision not to stand in a Mt Roskill byelection to give Labour's candidate Michael Wood a clear run.
It was the same kind of deal Seymour benefited from in 2014 in Epsom. Cue cries of hypocrisy from Seymour, who had been the target of derision from Wood when Wood stood in Epsom in 2014.
Cue return cries of hypocrisy from Green co-leader Metiria Turei.
Each then set about insisting their own "deal" was cleaner than the other's.
Turei insisted hers was cleaner because it was upfront. She put up the rather strained argument that it did not amount to asking Green voters to vote for Wood.
The reasoning for the deal was apparently not that they wanted Labour to win the seat, but that they did not want National to win it.
In return, Act and National argued their deal was clean because National had not withdrawn its candidate completely - simply asked their voters to support another candidate.
Turei's claim National and Act stitched up their deals with a "nod and wink" was true prior to 2014 - although the nods and winks were never exactly subtle. But in 2014, Key stood at his weekly press conference to announce his deals openly.
Key will not have forgotten that press conference, not least because he had the foresight to rule out helping Conservative Party leader Colin Craig.
Seymour will not have forgotten that press conference either. He has some licence to push back against National in carving his own niche. After all, picking up Act's vote enough to bring a second MP in with him would benefit them as well.
The 4 per cent that went to the Conservative Party last election is also ripe for the picking and Seymour and National want to ensure it does not all flow to Peters.
But when Key steps up to announce his 2017 deals, Seymour will be aware of the frequent and fickle fate of those miners' canaries.
