The answers you are reading are correct -- Eviolite Chansey does (usually) have higher Defence than Blissey with the same investment in its stat. You seem to have misunderstood how stat modifiers such as that which Eviolite provides are applied to a Pokemon's stats. They are not multiplied with the Pokemon's base stats the way you mentioned -- instead, they are an extension of the equation the game uses to determine Pokemon stats (the ones seen in a Pokemon's summary or on Showdown's teambuilder). Perhaps this is best explained by bringing up that equation here (credit to Bulbapedia):

The eqation above is used to determine every stat except HP, which is determined slightly differently. You may notice two things about this that relate to your question -- firstly, that base stats (represented by *Base*) are part of this equation, and secondly that there appears to be no place for any item modifier for boosts like Eviolite's 1.5x boost. This is because such modifiers, along with staged boosts (like those gained from Dragon Dance or lost from Indimidate) are considered *after* this calculation is made, and are not applied to the base stat which was already considered in the above eqation.

Bascially, Eviolite is not [Base stat] x 1.5, but instead [Full stat] x 1.5, which is why Eviloite Chansey beats Blissey in most instances. For example, if we took a Chansey and Blissey with the investment they both typcially use in competitive (252 Defence, Bold nature and 31 IVs), their stats would reach 119 and 130 respectively. However, 119 with Eviloite's 1.5x boost reaches 179 after rounding, which is about 37% better than Blissey. This is because Chansey's effective stat -- including its nature, base stats, IVs, EVs, level, etc. -- was multiplied by 1.5, not just its base stat.

It is worth noting, however, that Eviolite Chansey isn't *always* better, but for all competitive purposes, it's much better than Blissey. If Eviloite Chansey and Blissey each had their minimum defence stats (0 EVs, 0 IVs and a negative nature), Blissey would beat Eviolite Chansey by a few points (22 to 20 respectively, after rounding) but obviously this isn't reasonable in actual play.