Found a not entirely convenient solution.
I put the utilities I wrote into a separate typescript project, in this project I build them with type declaration generation. And assembled utilities are transferred to admin part. After that everything works as it should because it is already compiled JS with declarations.
But the problem still remains because it feels that when building the plugin strapi does not use tsconfig from plugin folder where target: ESNext is specified, but uses only its config from @strapi/typescript-utils/tsconfigs/admin, where target: ES5 is set