From the Pasuk , כי אל אשר תלכי אלך we see that Rus was Megayer after the death of her first husband.
If that is true then we need to understand why was it necessary to be Meyavem Rus? The first marriage was invalid.(she wasn't Jewish)
Answer I heard from Mori V’rabi Hagaon R. Shlomo Miller Shlita:
Rus was Megayer before her first marriage. This Giyur was assumed to be invalid because she accepted only 612 Mitzvohs.
She had not accepted The Mitzvah of not marrying a Moavi.
It was not yet known that a female Moavi may marry a Yisroel.
After the death of her husband she was Megayer a proper Giyur and accepted all 613 Mitzvohs. (כי אל אשר תלכי אלך)
Once Chazal were Mechadesh the Halacha מואבי ולא מואבית
this validated her first Giyur retroactively . She hasn’t rejected
a single Mitzvah and therefore her marriage was valid and required