IBM didn't want to rely solely on Intel when introducing PCs so it forced Intel to share its arch with another manufacturer that turned out to be AMD. It's not like AMD stole it. Math coprocessor was in turn invented by AMD (Am9511, Am9512) and licensed by Intel (8231, 8232).
They certainly didn't steal it. But Intel didn't second-source pentiums, or any chip with SIMD extension. AMD reverse-engineered those fair and square.
