Nonsense. Napoli were the Chelsea/Man City moneybags of that era. I don't remember Bournemouth breaking the world record transfer fee too many times. That's exactly what Napoli did when they bought Maradona. Brazilian stars Careca and Alemao weren't cheap either. Besides that, they were known for their defensive strength with established Italy internationals all over the team. Each Serie A title was largely down to defence, particularly in 86/87 when they only conceded 21 goals. Of course Maradona played a big part too, scoring 10 and 16 goals in these campaigns.
I would also strongly argue against it being the toughest league ever. In the 80s, 99% of the players in Serie A were Italian. It was a long way from the glory days of the mid-to-late 90s where the majority of top players from around the globe congregated in Italy. In Maradona's time, the top Serie A sides had 1/2 top foreigners, and the remainder of the squads were homegrown. Of course, it was a physical league if that's what you mean by tough.
Finally, (and I don't mean to bash Maradona, but Messi gets held to impossible standards, so why not?) Maradona only ever managed a cup in Spain, never produced anything in the European Cup with any club (biggest/most important club competition), and, oh yeah, is a drugs cheat after doping with ephedrine.