I'm reluctant since I think that through front-end coding the site can be optimzed for search engines.
Semantic markup, a.k.a. front-end coding, is a big part of SEO. Semantic markup is a bug signal as to what the content is that they are reading, what's important, etc. It's one of the biggest SEO factors a webmaster controls. The right markup can improve your rankings, poor markup can do the opposite.
Another guy told us that if we migrate platform we would lose all the current positioning on search engines.
This is true. Rankings are tied to URLs. Change your URL and that page is now brand new to the search engines. All incoming links and other SEO factors attributed to that page is gone. Fortunately you can tell the search engines that a page has moved and to make sure all the new URL gets all of the SEO credit the old URL had.
You have to do a 301 redirect from the old URL to the new URL. How you do this can vary depending on your site setup but if you search this site, or the Internet as a whole, you can find out how to do that. The catch is you have to do it for every page that changes URLs. Every page you skip starts over. And since internal linking and other pages mean each page on your site affects the others you don't want to be skipping any. And if there is no pattern to the new URLs from the old URLs this can be a tedious process. But it is well worth the effort.