This thread is dead-set bizarre. Unbelieveable amount of point-missing.
I'd probably agree that Steyn is at worst a contender for the "worst-ever best-seam-bowler-in-World", if you make the distinction that someone clearly was the best. The two periods mentioned - '33 to '39 and '65 to '69 - both probably contain there being no clear-cut best seamer around. John Snow was certainly the best in the latter and he was obviously > Steyn IMO. Ditto, currently and probably for good, Peter Pollock. Garth McKenzie'd have been the next one, and Steyn was clearly superior to him.
'33 to '39, though, as Mr Chandler says (though it might've been lost in the melee), there was obviously no seam-bowler playing who was a patch on Steyn. The best two were almost certainly Constantine and Martindale, who played just 17 Tests between them in that time. Both were terrific bowlers, and might perfectly possibly have been better than Steyn but I myself would say that anyone who tried to put forward a concrete case for any of the three would be guessing in the extreme. The most capped in the time were the Englishmen Farnes, Bowes and Allen and the Australian McCormick. And only Bowes of those even approaches top-class, the others were merely good, and I'm authoritively assured that Bowes was something of a seaming-deck bully, so he'd probably have to go down as inferior to Steyn.
If you make it very clear and obvious, and allow for no misunderstandings, you can make a case for "yes", but not a clear one:
"Including only times when there was one seam-bowler who was obviously the best around (and there'll have been plenty of times in Test history when there was no such thing), is Dale Steyn the worst of them?"
Just as a point of interest, we could perhaps say:
2008 to current - Steyn
2007/08 - Brett Lee
2006/07 - Mohammad Asif
2001 to 2005/06 - McGrath
1991/92 to 2000/01 - one of Waqar Younis, Ambrose, Wasim Akram, Donald and McGrath and never anyone else
1983 to 1991 - Marshall, and maybe sometimes Hadlee and Imran Khan
1970/71 - 1982/83 or so - Lillee and occasionally someone else, such as Roberts, Imran Khan or Holding
1966 to 1969 - at various points Snow and Peter Pollock
1962 to 1965 - Hall
1954 or so to 1961/62 - Trueman
1946 to 1953/54 - Lindwall
1933 to 1939 - Constantine or Martindale, or just possibly Bowes
1926 to 1932/33 - Larwood
And before that I'd not really be able to comment. Wickets were uncovered around The World and spin was almost always more important than seam in those days, so a truly outstanding seamer wasn't always absolutely neccessary for an excellent, never mind good, side.
Just to note - the above is certainly not exact in all cases, and would almost certainly require a nudge around here or there. But it's interesting to see that Lillee was almost certainly #1 for the longest time. Perhaps instructive as to why some people might think he was the best ever.