It does matter, which is why 1943-'45 is respected but it has a little bit of an asterisk because of the War and the fact that the Bentley's and the Kraut Line and Syl Apps and Turk Broda to name a few were off fighting. However, to a certain degree the NHL is the NHL.
But I do agree that you need to look at the eras and the competition. Here are each man's competition the years they finished 10th in scoring.
Sittler
1973-'74 - Esposito, Orr, Hodge, Cashman, Clarke, Martin, Apps Jr., Sittler, Hextall, Park
1975-'76 - Lafleur, Clarke, Perreault, Barber, Larouche, Ratelle, P. Mahovlich, Pronovost, Sittler, Apps Jr.
1976-'77 - Lafleur, Dionne, Shutt, MacLeish, Young, Perreault, Ratelle, McDonald, Sittler, Clarke
1977-'78 - Lafleur, Trottier, Sittler, Lemaire, Potvin, Bossy, O'Reilly, Perreault, Clarke, McDonald
1979-'80 - Dionne, Gretzky, Lafleur, Perreault, Rogers, Trottier, Simmer, Stoughton, Sittler, MacDonald
Alfredsson
2003-'04 - St. Louis, Sakic, Kovalchuk, Naslund, Hossa, Elias, Alfredsson, Stillman, Lang, Richards
2005-'06 - Thornton, Jagr, Ovechkin, Heatley, Alfredsson, Crosby, Staal, Kovalchuk, Savard, Cheechoo
2007-'08 - Ovechkin, Malkin, Iginla, Datsyuk, Thornton, Spezza, Zetterberg, Lecavalier, Alfredsson, Kovalchuk
Both lists have pretty tight competition at the top end.
With Sittler, he had 35/50 of his peers in those top 10 lists in the HHOF. (I included Sittler himself and counted the same player multiple times, same way for Alfredsson)
With Alfredsson there are 20/30 players who we project could be in the HHOF and I included some controversial ones like Kovalchuk and Lecavalier.
So that's 70% of Sittler's competition at the top were HHOFers while at best 66% of Alfredsson's peers are projected to be in the HHOF. That is barely any difference, and it just goes to show you that their eras are not that different and it wasn't much, if at all, easier for Sittler to stand out.