I've learned two languages with the help of MT. I'm sure you've interviewed people like me, but I get excited about the potential of MT for language learning, so I'd like to share my thoughts.
When I learned Spanish, I spent a lot of time chatting on Facebook with native speakers, and using Google Translate as "training wheels" to help me formulate sentences, and understand words and phrases I hadn't learned yet. It worked pretty well at the time (2012) except in cases of slang and typos that Google couldn't handle. I also used it a lot to help me translate blog posts from English to Spanish. Eventually, I graduated from the training wheels and was able to use Spanish fluently without the help of MT. More than once, while not using MT, I was told that I spoke Spanish with a "Google Translate accent", which I'm sure was more of a reference to my grammar than my accent, since my spoken practice was 100% with native speakers.
When I learned Hungarian (2019-now), at the beginning, Google Translate wasn't good enough to use for much more than getting a rough understanding of formal text, so I learned in a more traditional way at a school and with native speakers. Then the pandemic prevented me from doing both of those. I started chatting with native speakers on Facebook, but it was very difficult without MT and involved a lot of asking my conversation partners for translations and explanations. Progress was frustratingly slow. Then I discovered DeepL's MT, which was extremely good with Hungarian. I started using for chat conversations and emails, and people were shocked that I was managing to communicate with them so fluently. My progress of actually learning the language for myself accelerated dramatically. I've become conversational (B2/C1) in Hungarian in 2.5 years with very little in-person practice. Often, it takes native English speakers 5 years of in-person practice to reach that level. I'm convinced that MT played a key role in my ability to learn quickly.
When I use MT, I have a simple rule, that I have to understand each word of a translation before I send it. So I carefully read the translation, making sure that I understand each word. Sometimes that means I have to look up individual words/grammar before sending a message (I often use wiktionary for that, because it shows etymology), and other times, it means that I'll replace unfamiliar words or phrases in a translation with words and phrases from my own vocabulary. Over time I rely on MT less and less because my own vocabulary becomes stronger. I really believe that they key to learning a language quickly is to start USING the language as quickly as possible. Once you're using a language, your brain automatically starts picking up the skill. With traditional language learning, using a language can be very difficult in the beginning until you've reached a conversational level, but with MT, you can start using a language before you know everything.
For Spanish, I almost never use MT anymore. Sometimes I use it as a quick dictionary for an unfamiliar word, but my Spanish level is C2 and I use Spanish every day so it feels natural. I'm not ever translating in my head anymore.
For Hungarian, I'm still using MT often, but I don't need it during conversation (either written or spoken). Besides using it to translate things I don't know, I also find it useful for inputting Hungarian characters that are a pain to type with my US keyboard, and for conjugating words correctly when I know the root but am struggling for the correct ending. Often I'll know what I want to say in Hungarian, but I'll open DeepL and type in English, then adjust the translation to use the words I want before I copy and paste the Hungarian. I'm essentially using MT as a guide to help me craft my sentences even when I know what I want to say.
In summary, MT is awesome for language learning and for assisting language skills in development.