First of all, as a disclaimer : I wrote that article, and I've tried pretty-much everything related to Meteor SEO for the past year (server-side rendering with react, spiderable, serving static HTML with prerender).
has google improved its spider and now it should index my pages even without any improvement?
Believe me, it won't. You can give it a try tho, using the google search console with the option 'fetch the website like google bot', but google still isn't indexing JS-heavy webapps without work on your side
something free-forever
In the article, the solution isn't free-forever cause it's relying on prerender.io, which is a SAAS service, and only free for 200 pages in cache (Which is actually huge as a free plan, I've build several web apps with Meteor and haven't paid already).
BUT, if you build your own prerender-backend, which isn't that hard given that the code is actually open source, It will be free-forever, as long as you handle the back-end prerender service yourself.
It can be a pain, but will be somewhat free forever (I mean you'll only pay for your servers).
like 500px (500px.com), that are made with backbone, apparently without any improvement (try to disable js and it won't load the website), while the SEO is great as well.
Without improvement ?
500px are using rack-cache, and I'm am pretty sur they have a middleware to serve static HTML pages to crawlers, I'll fetch to website faking google bot to confirm this, but I am 90% sure they do.
So they are doing pretty much exactly what I am talking about in the article : Serving cached static HTML pages served by a middleware.
Good luck !