1. The United States always exempted yeshiva students. Respect for religion is ingrained in American culture. Even our current president has to pretend to be religious. It’s one the “exceptional” things that distinguishes us from Europeans or Israelis.
2. If someone was “yeshivish” frum, but not in yeshiva, the army would decide they were not suitable for the military. If he insisted on serving, they’ld find a “jobnik” (Israeli term) position for him. If you were willing to improvise and rely on heterim given for Jews serving in armies that executed people for refusing orders on religious grounds, it would be possible to serve in the military – a big factor is that America has never been as anti-religious as secular Israel.
3. The person who said “1941” doesn’t realize that while there was a limited draft in 1941, the US didn’t start serious conscription until 1942 (and in all fairness, probably could have relied on volunteers).
4. The person who said Canada didn’t read the “1941” – Canada entered the war in 1939, and had a policy of not sending conscripts overseas.